WIP: new travis setup with appraisal

This PR uses the new Travis build environment like provided by @joshk in #3595.
From now one we use [`appraisal`](https://github.com/thoughtbot/appraisal) to manage our gemfiles for different versions. The `RAILS` env variable no longer works.

TODOs:
- [ ] write some line in the docs about appraisal
- [ ] changelog entry
- [ ] make `script/locale` working with it