Migrate from TravisCI to GitHub Actions

[jamesaoverton]

The Travis tests broke a few days ago. It's nothing fundamental, just outdated versions of this and that. I tried to fix it in #751 but progress is slow. And we're almost out of free Travis credits anyway, so this seems like a waste of time.

We should move to GitHub Actions. I see two obstacles:

1. Unfortunately I think it will be harder than most projects to just translate the .travis.yml. To test the PURL system I set up Apache and then make all sorts of local HTTP requests. This is much more thorough than just testing that the YAML files look right. I was setting up Apache using Ansible. But I don't use Ansible for anything anymore. For local testing I was running Vagrant (on VirtualBox) then Ansible. I'm going to look into Docker + an install script instead.

2. This https://github.com/OBOFoundry/purl.obolibrary.org/blob/master/tools/safe-update.py script is specific to Travis, and we'll need something equivalent. Maybe gh can do most of this? @beckyjackson can you please look into that?

[jamesaoverton]

For (2) it looks like gh pr checks does what I want: https://cli.github.com/manual/gh_pr_checks. Despite the "pr" name, it seems to work on any branch.