travis: introduce multi-arch configuration

[mrc0mmand]

This PR supersedes #230 by moving testing into Travis CI, so we can test casync on multiple architectures. Currently it runs on:

• x86_64
• x86
• s390x
• ppc64le
• aarch64

It also uses Ubuntu Bionic instead of Xenial, so no more python 3.6 issues. After merging, the Semaphore CI for this repo can be turned off, but I'll pursue the migration to Semaphore v2 in other repos (mkosi & systemd).

/cc @evverx, @keszybz, @poettering

[evverx]

@mrc0mmand thank you!

As far as I know, travis_terminate should be called somewhere to prevent Travis from freezing when jobs fail (on arm at least). Could you try building casync with UBSan (so that the tests would fail) to figure out whether travis_terminate still should be used instead of exit?

[evverx]

Judging by https://travis-ci.org/systemd/systemd/jobs/639931506 (where it took Travis 10 minutes to terminate the job after it failed) it seems travis_terminate is still necessary.

[evverx]

One last thing. By the looks of it, the project isn't actively maintained so I'm not sure whether it makes sense to try to keep testing it (or try to additionally bring ASan+UBsan, Coverity and so on). I'd put the PR on hold and wait for @poettering to chime in.

[keszybz]

Damn. Travis is gone now too.

[keszybz]

Hmm, I think we want to keep this alive. @mrc0mmand do you think you could add the magic to switch to github actions?

[keszybz]

Scratch that. I'll talk with @poettering if we want to keep this project alive.