requirements: Test older versions of DRF (v2)

requirements: Test older versions of DRF (v2)

In commit febad055, we attempted to start testing older versions of
Django REST Framework for use with older versions of Django. This work
was later reverted in commit 6f2ddab6 due to issues with 'django-filter'
versions.

There are additional reasons for validating older versions of DRF, such
as supporting the versions provide as part of distributions. It turns
out that those issues aren't such a big deal and can be handled with a
shim. All in all, this means we can and should support these older
versions of Django REST Framework, and that is what we'll do.

Note that the minimum version supported in requirements.txt is not
changed as it's good practice to use the latest available version.
However, that doesn't make it any less supported.

Signed-off-by: Stephen Finucane <stephen@that.guru>