Migrate off of Travis CI (travis-ci.org shutting down)

[mtodd]

See https://blog.travis-ci.com/2021-05-07-orgshutdown for details on the shutdown of travis-ci.org.

Given builds here will inevitably stop running, we need to migrate builds off. Our best option is to migrate to GitHub Actions.

[djdefi]

Took a go at this in #109