tox: Add Django 1.11

tox: Add Django 1.11

Add support for the latest release of Django, 1.11. This is the next LTS
release (1.8 being the last one), so it's particularly important that we
maintain support for this going forward.

While neither the latest releases of django-rest-framework nor that of
django-filter explicitly support Django 1.11, it appears that they are
functional [1][2]. We can bump these packages separately when new
versions are released.

[1] https://github.com/encode/django-rest-framework/issues/5108
[2] https://github.com/carltongibson/django-filter/commit/1243ff7e

Signed-off-by: Stephen Finucane <stephen@that.guru>
Tested-by: Daniel Axtens <dja@axtens.net>