[ci] Move all CI off of Travis

[jameslamb]

Travis is putting much much stricter limits on free-tier use of its CI services.

https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing

For those of you who have been building on public repositories (on travis-ci.com, with no paid subscription), we will upgrade you to our trial (free) plan with a 10K credit allotment (which allows around 1000 minutes in a Linux environment).

We will be offering an allotment of OSS minutes that will be reviewed and allocated on a case by case basis. Should you want to apply for these credits please open a request with Travis CI support stating that you’d like to be considered for the OSS allotment.

The blog post also makes it sound like macOS jobs will no longer be available for free-tier users, but honestly the blog post's discussion of that is very confusing.

This project's CI should just be moved to GitHub Actions, to not have to deal with this.