Migrate ASAP from Travis CI as it no longer offers free OSS CI and will stop working very soon

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

We have been burning through our 10K credit for the last two weeks. When it runs out everything will just stop working unless we sign up for a paid account, but those do not look nearly as nice as what we had: $69/month gets just one concurrent job!

There seems to be a way to apply for a certain number of free OSS minutes every month but it sounds like we'd have to re-apply each month and it is not clear what the chance is of getting them nor whether we'd get enough to cover our usage.

I can't seem to find a way to view the current credit, so it's not clear when we'll run out, but I would imagine any minute.

Xref #4131 (closed) which lists possible alternatives of Github Actions and CircleCI. Or Appveyor in a reversal of that issue.