PHPUnit 8 adds `void` return type declarations to the PHPUnit methods used by PHPCS, making it neigh impossible to make the unit test suite cross-version compatible for the PHP versions supported by PHPCompatibility.

As of the recent update to the Travis images, the images for PHP 7.2+ now ship with PHPUnit 8 which causes the unit tests to fail.

Officially, PHPUnit 7 is compatible with PHP 7.1, 7.2 and 7.3.
Ref: https://phpunit.de/supported-versions.html

So, for Travis images which come natively with PHPUnit 8 (PHP >= 7.2), PHPUnit 7 needs to be installed via Composer.

As PHP 7.3 has been released, the build to test code coverage should be run on PHP 7.3.

This should raise the code coverage as for, for instance, the `FlexibleHeredocNowdoc` sniff, code coverage for a significant part of the sniff will only show up when run on PHP 7.3.

In contrast to when the PHP 7.3 build was added, there does appear to be a code coverage driver available for PHP 7.3 by now, so we should be safe to make this change now.

Ref: 764

There is no PHPUnit version which is currently compatible with PHP 8.
As that either means that the builds for nightly would always fail or - if the unit tests would be skipped -, the only check executed on nightly would be linting the files, I've elected to remove build testing against nightly for the time being.

For more details about PHPUnit vs PHPCS vs PHP 8, see squizlabs/PHP_CodeSniffer 2416