Travis: various minor tweaks

[jrfnl]

Context

• Improve CI setup

Summary

This PR can be summarized in the following changelog entry:

• Improve CI setup

Relevant technical choices:

• Be explicit about the expected OS.
• Cache composer downloads for faster builds.
• Use the jobs key. The matrix key has been deprecated quite a while back.
• Use fast_finish.
  This will mark a build as "passed" as soon as all builds which are not allowed to fail have passed.
• Disable Xdebug when it's not needed.
  Xdebug is only needed for code coverage builds and slows down the Composer install and test runs significantly, so conditionally disabling it.
• Use travis_retry for the composer install to prevent failed builds due to time-outs and rate limits.
• Validate the composer.json file just to be sure.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Verify the output of the Travis build.
• Check the "View config" tab in Travis to see only one warning left, instead of the previous three.
• Take note of the speed increase in the PHP 7.4 build, which is currently the only build which runs the tests + PHPCS. I realize it's only ~12 seconds (~15%), but when the tests will be enabled for more builds, those seconds still add up ;-)

[enricobattocchi]

Great! Merging