Stick to Django support's roadmap

[berinhard]

As listed here, Django's versions lower than 1.11 aren't supported anymore.

Since there are a lot of versioning conditions in model mommy's code, I really think it would be good to also drop django's 1.8, 1.9 and 1.10 version support and to clean up the code a little bit.

[anapaulagomes]

I'm on it.