Migrate to GH Actions from Travis, drop old PHP versions, support PHP 8

[Seldaek]

Unfortunately the test suite fails for some reason I don't really understand. Hopefully this branch can be used as a base by someone else more familiar with the code though.

[sc0rp10]

oh my god, this is seldaek himself, wow!

[biggianteye]

Thanks for trying this out. It looks like the tests are failing because of dependencies that need updating. This is all a bit of a versioning nightmare. However having this PR will be a useful reference.