PHP 8 support

[Ayesh]

• Allow PHP 8 in composer.json
• Allow PHPUnit 7.0 through 9.3. PHPUnit 9.3 supports PHP 8.0 and is the only official lowest version to do so.
• Prophecy is currently not allowing PHP 8.0, hence this being a draft PR.

[Ayesh]

Hi @l0gicgate - thanks a lot for reviewing this. This PR was made before PHPUnit 9 had PHP 8 support in composer.json, and before PHPUnit 8.5 fixed compatibility issues with PHP 8. So I had to use ignore-platform-reqs to bypass Composer validating them.

I removed them, and will mark this PR as ready.

Travis internally does not provide PHP 8 under trusty (14 LTS), so I also updated to xenial (16 LTS) for PHP 8 builds, which now supports all PHP versions we test against.

All tests now pass as well.

Thank you.

[akrabat]

Thanks for your work on this. Due to our tardiness, there were multiple PRs raised and I picked #71.