attempting a more elegant solution to detecting django version

Never done this version check before, but this certainly seems like better logic than the comparable string comparison:

```
if django.get_version() >= "1.7.0":
   ...

```

Not sure if this will successfuly close: https://github.com/comoga/django-constance/issues/80, but it's a start I suppose!
This would fail on version 2.0.