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.
test_requirements: do not pin selenium to 2.37.2

I was seeing the following error with selenium tests, which is fixed in
a later version:

> WebDriverException: Message: "Can't load the profile. Profile Dir: …"
Merge pull request #302 from blueyed/tests-do-not-pin-selenium

test_requirements: do not pin selenium to 2.37.2
Merge pull request #299 from blueyed/fix-tox-ini-django-twice

tox.ini: do not install `django` twice
Use `queryset` from a ModelChoiceField for `choices`

Fixes https://github.com/yourlabs/django-autocomplete-light/issues/300
Sort/cleanup imports