Green lights in Travis

This pull request contains the following changes, not all of them related:

- Green lights in Travis: https://travis-ci.org/matthiask/django-mptt
- A modification of `.travis.yml` so that django-mptt is not only tested with different Python versions, but also with all supported Django versions.
- A mostly flake8-clean code base except for E128 (indentation) and E501 (line length). We can remove those warnings later.
- Removal of FeinCMS-specific code.
- And last (but not least) a potentially controversial fix for the `get_query_?set` manager methods mess. I'd argue that we do not need to provide any code handling those two methods ourselves as long as we do not use either of them. We can simply using `.filter()`, `.all()` etc. instead and let Django do the work of calling the correct `get_query_?set` method. It's not our job to be smarter than Django's maintainers. Users of django-mptt should read Django's release notes and adapt their code to use the current convention at a convenient time.

(Not pushing this myself because these changes really need a second pair of eyes.)
Here, to preserve indentation, you can use `textwrap.dedent`