Integrate Credo into travis

[NobbZ]

Since we have decided in #12 that we want an error on errornous typestuff in dialyzer, I thought that we should think about also using credo in travis.

This would again mean, that emitted credo warnings will result in travis fails. Therefore I do think, that we should discuss this here, and regardless of the outcome of this discussion, we should not integrate it before we haven't finished repairing all the typestuff coming from dialyzer integration.