Use GitHub actions for continuous integration (CI)

[SimonFrings]

Bye bye Travis CI, you've served us well.
This code supports running tests on PHP 8.

$ docker run -it --rm -v `pwd`:/data --workdir=/data php:8.0.0 php vendor/bin/phpunit
PHPUnit 9.5.4 by Sebastian Bergmann and contributors.

............S...                                                  16 / 16 (100%)

Time: 00:00.376, Memory: 6.00 MB

OK, but incomplete, skipped, or risky tests!
Tests: 16, Assertions: 33, Skipped: 1.

Builds on top of #13 and reactphp/reactphp#442.

[clue]

@SimonFrings Thanks for the update, changes LGTM!