Replace travis-ci with Github Actions

[Geod24]

This will be a bit problematic for Buildkite.

@s-ludwig : See https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing and the result: https://www.jeffgeerling.com/blog/2020/travis-cis-new-pricing-plan-threw-wrench-my-open-source-works

This only does Ubuntu, as currently we can't merge any PR because a "Required" check (Travis) is not working anymore.
In the future I will try to add Mac.

[s-ludwig]

Probably makes sense to also add Windows (as for vibe-core) at some point. Seems like the workflow hasn't generated any jobs, though.

[Geod24]

Yeah, I wanted to add macOS / Windows, but since all PRs are being blocked from being merged, I figured I'd start slow :)
You don't see the checks in this PR because it wasn't merged. However, you can see the run in my fork: https://github.com/Geod24/vibe.d/actions/runs/382413068

There's still the question of the failing test. For some reason, the TLS client tunnel succeeds when a failure is expected. I assume that's because of configuration difference between the new mongodb/redis and the old one (e.g. the old one didn't support TLS?). It would be great if you could take a look.

[s-ludwig]

Okay, I'll merge like this and then take a look at the test, so that I can actually see the failure.