In #26, we began adding a require statement to the package's composer.json:

The minimum version declaration is also used in wp package install, which will err if the minimum version isn't met.

However, because bin/install-package-tests.sh calls composer install --dev, WP-CLI is unnecessarily installed a second time via Composer:

https://travis-ci.org/wp-cli/find-command/jobs/209460745

We should get this fixed, and apply it to all of the packages.