... and remove the `composer self-update` from the Travis script as it's no longer needed.

YoastCS 2.1.0 requires the DealerDirect Composer PHPCS plugin at version `0.7.0`, which is compatible with Composer 2.0.

Refs:
* https://github.com/Yoast/yoastcs/releases/tag/2.1.0

.. to simplify handling of those files which don't need PHPUnit version based loading.

The `default` case translates the class name to the file path based on PSR4 and loads them, without using hard-coded class names nor file paths.

This ensures that the "forward compatible" namespaced versions of common PHPUnit classes/interfaces are available.

PHP 6.0 introduced namespaces to PHPUnit.
The last few version(s) of PHPUnit 4.x and 5.x, contain both non-namespaced and namespaced versions of the most commonly referred to classes used by test suites.

By being strict about the minimum supported versions of PHPUnit 4.x and 5.x, we can be sure that the namespaced versions of the files are available when we need them and that we don't need to create `class_alias()`-es for these.

Add cross-version handling of the `assertPreConditions()` and `assertPostConditions()` methods via snake_case methods to the `Yoast\PHPUnitPolyfills\TestCases\TestCase`.

Includes unit tests.

Includes improving the documentation within the class.

Note: these methods are not included in the `XTestCase` as there are no annotations available to handle these methods.

The PHPUnit `assertAttribute*()` methods were deprecated in PHPUnit 8.x and removed in PHPUnit 9.0.

Public properties can still be tested by accessing them directly.
Protected and private properties can no longer be tested using PHPUnit native functionality.

The reasoning for the removal of these assertion methods is that _private and protected properties are an implementation detail and should not be tested directly, but via methods in the class_.

It is strongly recommended to refactor tests, and if needs be, classes to adhere to this.

However, if for some reason the value of protected or private properties still needs to be tested, this helper can be used to get access to their properties and value.

Includes tests.

Fixes 2

The method signatures in the PHPUnit `TestListener` interface have changed a number of times across versions, including adding scalar parameter (PHP 7.0+) and `void` return type (PHP 7.1+) declarations.

Additionally, the use of the TestListener principle has been deprecated in PHPUnit 7 in favour of using the [TestRunner hook interfaces](https://phpunit.readthedocs.io/en/9.3/extending-phpunit.html#extending-the-testrunner).

If a test suite does not need to support PHPUnit < 7, it is strongly recommended to use the TestRunner hook interfaces extensions instead.

However, for test suites that still need to support PHPUnit 6 or lower, implementing the `TestListener` interface is the only viable option.

This commit adds a `Yoast\PHPUnitPolyfills\TestListeners\TestListenerDefaultImplementation` trait which overcomes the signature mismatches by having multiple versions and loading the correct one depending on the PHPUnit version being used.

Similar to the `TestCase` implementation, snake_case methods without type declarations are used to get round the signature mismatches. The snake_case methods will automatically be called.

Includes tests for this TestListener implementation.

Includes making an allowance for the `float` parameter and `void` return type in the PHPUnit 7+ specific testlistener trait in:
* the PHPCS config.
* the Parallel Lint command.

... and allow this package to be installed on PHP 5.5.

…methods

PHPUnit 5.0.0 introduced the new `Assert::assertFinite()`, `Assert::assertInfinite()` and `Assert::assertNan()` methods.

This commit:
* Adds two traits with the same name.
    One to polyfill the methods when not available in PHPUnit.
    The other - an empty trait - to allow for `use`-ing the trait in PHPUnit versions in which the methods are already natively available.
* Logic to the custom autoloader which will load the correct trait depending on the PHPUnit version used.
* An availability test for the functionality polyfilled.

Note: the methods use `static::` to call the PHPUnit native functionality. This allows for existing method overloads in a child class of the PHPUnit native `TestCase` to be respected.

PHPUnit 5.2.0 introduced the new `TestCase::expectException()`, `TestCase::expectExceptionCode()`,`TestCase::expectExceptionMessage()` and `TestCase::expectExceptionMessageRegExp()` methods as a replacement for the `Testcase::setExpectedException() method which was deprecated in PHPUnit 5.2.0 and the  `Testcase::setExpectedExceptionRegExp()` method which was deprecated in 5.6.0.
Both these methods were removed in PHPUnit 6.0.0.

This commit:
* Adds two traits with the same name.
    One to polyfill the methods when not available in PHPUnit.
    The other - an empty trait - to allow for `use`-ing the trait in PHPUnit versions in which the methods are already natively available.
* Logic to the custom autoloader which will load the correct trait depending on the PHPUnit version used.
* An availability test and partial functionality test for the functionality polyfilled.

Includes:
* Adding the new polyfill to the existing `TestCases` classes.
* Adding the new polyfill to select test files to allow those to run and pass on PHPUnit 4.8.x.
* Making an allowance in the PHPCS ruleset for an exception quirk which PHPCS can't handle correctly.

… al methods

PHPUnit 5.6.0 introduced the new `Assert::assertIsReadable()`, `Assert::assertNotIsReadable()`, `Assert::assertIsWritable()`, `Assert::assertNotIsWritable()`, `Assert::assertDirectoryExists()`, `Assert::assertDirectoryNotExists()`, `Assert::assertDirectoryIsReadable()`, `Assert::assertDirectoryNotIsReadable()`, `Assert::assertDirectoryIsWritable()`, `Assert::assertDirectoryNotIsWritable()`, `Assert::assertFileIsReadable()`, `Assert::assertFileNotIsReadable()`, `Assert::assertFileIsWritable()`, and `Assert::assertFileNotIsWritable()` methods.

This commit:
* Adds two traits with the same name.
    One to polyfill the methods when not available in PHPUnit.
    The other - an empty trait - to allow for `use`-ing the trait in PHPUnit versions in which the methods are already natively available.
* Logic to the custom autoloader which will load the correct trait depending on the PHPUnit version used.
* An availability test for the functionality polyfilled.

Note: the methods use `static::` to call the PHPUnit native functionality. This allows for existing method overloads in a child class of the PHPUnit native `TestCase` to be respected.

Includes:
* Adding the new polyfill to the existing `TestCases` classes.
* Adding the new polyfill to select test files to allow those to run and pass on PHPUnit 4.8.x.
* Making an allowance in the PHPCS ruleset for an exception quirk which PHPCS can't handle correctly.

To allow for testing on PHP 5.5, PHPUnit 4 is needed.

PHPUnit 4.8.36 is the only version in the 4.x series which contains the forward-compatible layer with the namespaced versions of common PHPUnit classes.

With the additional polyfills which have been put in place, installation of this package on PHP 5.5 in combination with PHPUnit 4.8.36 is now supported.

Includes annotating this change in all the relevant places.
Includes adding a new build to the Travis matrix to also run all tests against PHP 5.5 / PHPUnit 4.8.36.