Allow PHPUnit 7.x and PHP 7.3

[mimmi20]

This PR want the following:

• allow tests on PHP 7.3 on travis-ci (nightly is now 7.4-dev)
• allow tests with PHPUnit 7.x
• use the build stages on travis-ci to avoid if commands
• test against --prefer-lowest for every PHP version
• remove xdebug for builds without coverage to make them faster

This is nessesary to allow PHP 7.3 and PHPUnit 7.5 for PHP-DI

[mnapoli]

Thanks!