Multi-stage Travis builds

[Felix Breidenstein]

Would be great to have different steps in TravisCI like lint/build/test/docker/deploy with dependencys on each other so we break out early when things go wrong.

Maybe @thilobillerbeck ,our good old TravisCI-Expert, is up for a grab? ;)

[Thilo Billerbeck]

I think calling me an expert is an overstatement, but I can help here 👍