Allow PHP 8, move tests to GitHub Actions

[aleksblendwerk]

Short summary:

• Allows installation on PHP 8
• Moves CI to GitHub Actions, runs tests for PHP 7.3, 7.4 and 8.0
• Sets the PHPUnit version installed by simple-phpunit to 9.4 in phpunit.xml.dist
• The actual bundle or test code hasn't been touched at all

The badge in README.md should display right when this is merged - but from what I can see it will always display the status for the master branch - not sure if this was any different using Travis CI. There is a way to manually change the branch the badge refers to for long-running branches though.

I have also made a quick test using this version within pingcrm-symfony on PHP 8.0 and all seems fine.

[aleksblendwerk]

Hmm. I am not sure if the proposed GitHub Actions workflow was supposed to show up here as a check already. Might only be activated after you agreed on merging the PR. It is set to run on every push and PR.

[rompetomp]

Thanks Aleks, looking good, like always! I'll merge the PR, and we'll see if it will run the Github action :D

[aleksblendwerk]

Looking good, nice! 😇

Also: you are the fastest PR merger and new release builder ever! Thanks! 🚀