Pivot to GitHub Actions

[leoarnold]

In late 2020, Travis CI decided to discontinue unlimited free builds for public projects. While they still hand out free build minutes for opensource project, they do this through an application process, thereby making it difficult to quickly contribure:

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

Since GitHub Actions still allows unlimited build time for public project, it became the go-to CI for opensource projects on GitHub.

We also update the test build to include Rails 6 and 7.