Rubocop on travis

The Idea is to ensure that future changes do not add new offences to the rubocop checks. For this all existing offences needed to be solved or disabled for now. Its not perfect but the big benefit is, that future changes are checked automatically. Without a regular check on travis somebody has so check rubocop from time to time manually.

@sds let me know what you think about this. I split everything into small commits so feel free to tell me what you think ab them and if I should change anything.