Revert "requirements: Test older versions of DRF"

Revert "requirements: Test older versions of DRF"

This partially reverts commit febad055fb6609369f1a465a5eec323549c5c065.
While Django REST Framework works with Django 1.6 and 1.7, the versions
of Django Filters that provide DRF integration do not [1]. It doesn't
really make sense to enable only partial REST API support (i.e. no
filtering) for users with older Django versions and this approach will
cause far too much confusion among users. Better to just drop REST API
support for users with these insecure versions and encourage them to
update to supported versions of Django should they wish to use these
features.

[1] https://github.com/carltongibson/django-filter/blob/0.15.0/CHANGES.rst

Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: 0fc32337 ("REST: Integrate django-filter support")