Replace Travis CI with GitHub Actions

[ocean90]

Since Travis CI no longer runs builds for open source projects, the current .travis.yml config needs to be converted to one or more GitHub workflows.

With the switch we should also make sure that all current PHP versions are tested.

[jrfnl]

I'm looking into this.

An initial finding is that PHP 5.2 would not be easy to test anymore as it is not supported by the best "setup PHP" action.

Personally, I'd advocate for dropping support for PHP 5.2 by adding a Requires PHP: 5.3 header and no longer testing on PHP 5.3.