tox.ini: do not install `django` twice

Fixes:
> Double requirement given: django (from -r test_project/requirements.txt (line 3)) (already in django, name='django')"
tox.ini: set DJANGO_SETTINGS_MODULE=test_project.settings

This is useful/required, if `DJANGO_SETTINGS_MODULE` is set in the
environment already, and `os.environ.setdefault` is used for the
test_project.
Merge pull request #299 from blueyed/fix-tox-ini-django-twice

tox.ini: do not install `django` twice