Move to GitHub actions

[alexpott]

Move away from travis to make testing on different PHP versions simpler and better integration with github. This will allow us to add PHP 8.1 testing in #384

[alexpott]

You can see the actions in action here - https://github.com/alexpott/easyrdf/actions/runs/1311440366

[osma]

Great work! Much appreciation!

You could consider adding PHP 8.1 tests here as well in experimental mode (allowed to fail with continue-on-error enabled just for that version), like this:

strategy:
  matrix:
    php: [ '7.1', '7.2', '7.3', '7.4', '8.0' ]
    experimental: [false]
    include:
       - php: '8.1'
         experimental: true
  fail-fast: false
  continue-on-error: ${{ matrix.experimental }}

This would highlight the errors encountered with PHP 8.1 without breaking the build. Then the experimental rule can be again dropped in PR #384 (but perhaps reintroduced later when 8.2 is released...)

But this is no big deal - simply merging this first and then merging PR #384 (with PHP 8.1 added to the matrix) as you suggested would give the same result.