Migrate ALE from Travis CI to Github Actions

[hsanson]

Travis-CI.com no longer provides unlimited credits to open source projects:

• https://www.jeffgeerling.com/blog/2020/travis-cis-new-pricing-plan-threw-wrench-my-open-source-works

And ALE already ran out of credits, therefore there are many PR's stuck waiting for checks from travis that will never complete. One way to solve this is to migrate from Travis-CI to Github actions:

• https://fbernutz.github.io/posts/github-actions

Since ALE used Docker to run tests we can use these actions for the migration:

• https://github.com/marketplace/actions/docker-run-action
• https://github.com/marketplace/actions/build-and-push-docker-images

[w0rp]

I have merged this now, and I updated the project settings to turn off Travis CI checks anywhere I could find them and require the GitHub checks instead.