Remove django from `install_requires`

Having `django` in `install_requires` makes it impossible to try out a Django release candidate or dev branch. My requirements file is something like this:
```
https://www.djangoproject.com/download/1.7c1/tarball/
django-docopt-command==0.1.0
```

But when I `pip install -r requirements.txt` it first installs Django 1.7c1, then `django-docopt-command` tries to being in Django 1.6.5:
```
Downloading/unpacking django (from django-docopt-command==0.1.0->-r requirements/_common.txt (line 8))
```
This unfortunately trashes my existing Django install: I see this output at some point:
```
    You have just installed Django over top of an existing
    installation, without removing it first. Because of this,
    your install may now include extraneous files from a
    previous version that have since been removed from
    Django. This is known to cause a variety of problems. You
    should manually remove the
    
    /home/paul/.virtualenvs/api3/lib/python2.7/site-packages/django
    
    directory and re-install Django.
```

For reference, other Django packages don't specify a requirement on Django for installation, and I think this is the correct thing to do:

- https://github.com/tomchristie/django-rest-framework/blob/master/setup.py
- https://github.com/kennethreitz/dj-static/blob/master/setup.py