Travis: simplify the script
Travis: simplify the script

Current situation:
* The Travis native PHPUnit is used in most cases, though on PHP > 7.1, a PHPUnit phar is downloaded.

New situation:
* Always installed PHPUnit with Composer.
    For PHP 8 (`nightly`) ignore platform requirements to allow PHPUnit 7.x to install.

This makes us less dependent on the Travis images being correct (we've seen problems before aka the current special casing) and as the Travis cache should retain the downloaded Composer packages, this should really have much impact on the build time and gives us more flexibility to add other dev tools.

WordPress-Coding-Standards/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards,WordPress-Coding-Standards/WordPress-Coding-Standards