Run gunicorn from the django virtualenv if django is there.

This lets us follow the following guidance on the Django web site:

"If you are using Django 1.4 or newer, it’s highly recommended to simply
run your application with the WSGI interface using the gunicorn command
as described above."

(from https://docs.djangoproject.com/en/1.6/howto/deployment/wsgi/gunicorn/#using-gunicorn-s-django-integration )