Improve test runner configuration

[lorddaedra]

I have several ideas how we can improve our test configs:

• Add Python 3.6 support
• Remove Python 3.3 support (eol, see pep-0398)
• Remove Django 1.9 support (eol, see https://www.djangoproject.com/download/, bottom of page)
• Add Django 2.0b1, allow failures for now (https://docs.travis-ci.com/user/customizing-the-build/#Rows-that-are-Allowed-to-Fail)
• Replace ==version to intervals like “django>=1.11,<2.0” (to get latest 1.11.x version for testing), it's good idea to always use latest bugfix versions for tests