Replace Travis config with gitlab-ci config

Using a super fast Alpine Linux image. The style check had to be removed, because it was cloned from craftyguy's github repository which does not exist anymore. This CI script also builds the source.

@craftyguy: when you have time, could you take a look at this? Also I've been thinking about replacing the linting code with something we can directly install from Alpine's repos, like clang-format (part of clang)? (Depending on the effort this takes, we might want to do that in a follow up merge request.)

EDIT: he wrote me, that he will be busy over the next ~10 days, so it would be nice if someone else could review it. I recommend that we get basic CI going with this merge request, and then possibly look into automatic code formatting afterwards.

Fixes #84 (closed)