Migrate to GitHub actions

[nickevansuk]

Set up CI in the PHP lib to use GitHub actions match what’s in OpenActive.NET:

• Runs the model generator (similar to this https://github.com/openactive/OpenActive.NET/blob/master/.github/workflows/create-data-model-pr.yaml), to create a new PR when the data models update
• Run tests on all new PRs (similar to this https://github.com/openactive/OpenActive.NET/blob/master/.github/workflows/tests.yaml) - a migration from Travis

[nathansalter]

I like this idea, looks like Travis are slowly squeezing their OSS users out, so should protect us in the future

[nickevansuk]

This has now been done!