Require Django dep.

This wouldn't build on a fresh environment due to Django being required for the build.

```
pip install django-sirtrevor
  File "/vagrant/venv/build/django-sirtrevor/setup.py", line 3, in <module>
    from sirtrevor import __version__
  File "sirtrevor/__init__.py", line 2, in <module>
    from django.template.loader import render_to_string
ImportError: No module named django.template.loader
```