Migrate from Travis CI to GitHub Actions

Travis CI recently announced
[changes to its pricing model](https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing)
that will affect OSS projects. Whereas Travis previously allowed unrestricted
hours for OSS projects to run on their build machines, the new pricing model
will require the use of credits, of which free plans only have a limited
quantity. Moreover, Travis now takes an extraordinarily long time to actually
serve CI requests, which is infeasible in the long term.

As a result, I have decided
(with encouragement from other Haskell maintainers, see
[here](https://old.reddit.com/r/haskell/comments/jvbs49/psa_maintainers_should_think_about_migrating_off/))
to migrate to GitHub Actions. This
relies on a very recent version of `haskell-ci` that features a GitHub Actions
backend.