Fix warning on Django 1.8 and prepere for 1.9

**django_settings/models.py:5:**

```
RemovedInDjango19Warning: django.contrib.contenttypes.generic is deprecated and will be removed in Django 1.9. Its contents have been moved to the fields, forms, and admin submodules of django.contrib.contenttypes.
from django.contrib.contenttypes import generic
```
