Remove support for Django 1.6, 1.7

Remove support for Django 1.6, 1.7

These versions are massively outdated and the only reason for keeping
them was to allow installation on RHEL 7 using the version provided via
EPEL. No one's actually using this so just kill it.

This also allows us to remove support for django-filter 0.11, which was
only retained for use with these older versions of Django.

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