chore(deps): update dependency mocha to v6

This PR contains the following updates:

| Package | Type | Update | Change | References |
|---|---|---|---|---|
| mocha | devDependencies | major | `^5.2.0` -> `^6.0.0` | [homepage](https://mochajs.org/), [source](https://togithub.com/mochajs/mocha) |

---

### Release Notes

<details>
<summary>mochajs/mocha</summary>

### [`v6.0.0`](https://togithub.com/mochajs/mocha/blob/master/CHANGELOG.md#&#8203;600--2019-02-18)

[Compare Source](https://togithub.com/mochajs/mocha/compare/v5.2.0...v6.0.0)

#### 🎉 Enhancements

-   [#&#8203;3726](https://togithub.com/mochajs/mocha/issues/3726): Add ability to unload files from `require` cache ([**@&#8203;plroebuck**](https://togithub.com/plroebuck))

#### 🐛 Fixes

-   [#&#8203;3737](https://togithub.com/mochajs/mocha/issues/3737): Fix falsy values from options globals ([**@&#8203;plroebuck**](https://togithub.com/plroebuck))
-   [#&#8203;3707](https://togithub.com/mochajs/mocha/issues/3707): Fix encapsulation issues for `Suite#_onlyTests` and `Suite#_onlySuites` ([**@&#8203;vkarpov15**](https://togithub.com/vkarpov15))
-   [#&#8203;3711](https://togithub.com/mochajs/mocha/issues/3711): Fix diagnostic messages dealing with plurality and markup of output ([**@&#8203;plroebuck**](https://togithub.com/plroebuck))
-   [#&#8203;3723](https://togithub.com/mochajs/mocha/issues/3723): Fix "reporter-option" to allow comma-separated options ([**@&#8203;boneskull**](https://togithub.com/boneskull))
-   [#&#8203;3722](https://togithub.com/mochajs/mocha/issues/3722): Fix code quality and performance of `lookupFiles` and `files` ([**@&#8203;plroebuck**](https://togithub.com/plroebuck))
-   [#&#8203;3650](https://togithub.com/mochajs/mocha/issues/3650), [#&#8203;3654](https://togithub.com/mochajs/mocha/issues/3654): Fix noisy error message when no files found ([**@&#8203;craigtaub**](https://togithub.com/craigtaub))
-   [#&#8203;3632](https://togithub.com/mochajs/mocha/issues/3632): Tests having an empty title are no longer confused with the "root" suite ([**@&#8203;juergba**](https://togithub.com/juergba))
-   [#&#8203;3666](https://togithub.com/mochajs/mocha/issues/3666): Fix missing error codes ([**@&#8203;vkarpov15**](https://togithub.com/vkarpov15))
-   [#&#8203;3684](https://togithub.com/mochajs/mocha/issues/3684): Fix exiting problem in Node.js v11.7.0+ ([**@&#8203;addaleax**](https://togithub.com/addaleax))
-   [#&#8203;3691](https://togithub.com/mochajs/mocha/issues/3691): Fix `--delay` (and other boolean options) not working in all cases ([**@&#8203;boneskull**](https://togithub.com/boneskull))
-   [#&#8203;3692](https://togithub.com/mochajs/mocha/issues/3692): Fix invalid command-line argument usage not causing actual errors ([**@&#8203;boneskull**](https://togithub.com/boneskull))
-   [#&#8203;3698](https://togithub.com/mochajs/mocha/issues/3698), [#&#8203;3699](https://togithub.com/mochajs/mocha/issues/3699): Fix debug-related Node.js options not working in all cases ([**@&#8203;boneskull**](https://togithub.com/boneskull))
-   [#&#8203;3700](https://togithub.com/mochajs/mocha/issues/3700): Growl notifications now show the correct number of tests run ([**@&#8203;outsideris**](https://togithub.com/outsideris))
-   [#&#8203;3686](https://togithub.com/mochajs/mocha/issues/3686): Avoid potential ReDoS when diffing large objects ([**@&#8203;cyjake**](https://togithub.com/cyjake))
-   [#&#8203;3715](https://togithub.com/mochajs/mocha/issues/3715): Fix incorrect order of emitted events when used programmatically ([**@&#8203;boneskull**](https://togithub.com/boneskull))
-   [#&#8203;3706](https://togithub.com/mochajs/mocha/issues/3706): Fix regression wherein `--reporter-option`/`--reporter-options` did not support comma-separated key/value pairs ([**@&#8203;boneskull**](https://togithub.com/boneskull))

#### 📖 Documentation

-   [#&#8203;3652](https://togithub.com/mochajs/mocha/issues/3652): Switch from Jekyll to Eleventy ([**@&#8203;Munter**](https://togithub.com/Munter))

#### 🔩 Other

-   [#&#8203;3677](https://togithub.com/mochajs/mocha/issues/3677): Add error objects for createUnsupportedError and createInvalidExceptionError ([**@&#8203;boneskull**](https://togithub.com/boneskull))
-   [#&#8203;3733](https://togithub.com/mochajs/mocha/issues/3733): Removed unnecessary processing in post-processing hook ([**@&#8203;wanseob**](https://togithub.com/wanseob))
-   [#&#8203;3730](https://togithub.com/mochajs/mocha/issues/3730): Update nyc to latest version ([**@&#8203;coreyfarrell**](https://togithub.com/coreyfarrell))
-   [#&#8203;3648](https://togithub.com/mochajs/mocha/issues/3648), [#&#8203;3680](https://togithub.com/mochajs/mocha/issues/3680): Fixes to support latest versions of [unexpected](https://npm.im/unexpected) and [unexpected-sinon](https://npm.im/unexpected-sinon) ([**@&#8203;sunesimonsen**](https://togithub.com/sunesimonsen))
-   [#&#8203;3638](https://togithub.com/mochajs/mocha/issues/3638): Add meta tag to site ([**@&#8203;MartijnCuppens**](https://togithub.com/MartijnCuppens))
-   [#&#8203;3653](https://togithub.com/mochajs/mocha/issues/3653): Fix parts of test suite failing to run on Windows ([**@&#8203;boneskull**](https://togithub.com/boneskull))

</details>

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