Migrate CI away from from Travis CI

I have been waiting for CI checks in #40, that resolves #39, to complete for about 15 days. However it always shows Expected — Waiting for status to be reported.

[travisdowns]

The free ride is over, I guess:

[nemequ]

It's not as simple as changing from .org to .com, unfortunately. I would have to contact them about providing some credits to zero out the balance for my account, and even then I'd only be willing to compile on POWER and Arm because those don't consume credits. That means that we wouldn't be able to run a lot of the builds.

The right solution here is really to switch to alternative providers. GitHub Actions is generally the best option, but if more are necessary there is Cirrus CI, Circle CI, AppVeyor, Azure Pipelines, Drone.io, and more.

The only real reason I see to ever use Travis CI these days is for POWER and s390x builds, for which they're pretty much the only option other than an emulator. Travis CI also supports Arm, but so does Drone.io. Given the wealth of much better choices out there, I'm not at all tempted to add any new x86 builds on Travis CI.