Migrate to GitHub Actions

[Travis CI has changed their pricing model](https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing) and it's [no longer usable for OSS projects](https://www.jeffgeerling.com/blog/2020/travis-cis-new-pricing-plan-threw-wrench-my-open-source-works).

I would suggest migrating CI to GitHub Actions.