Add support for Django REST Framework 3.7 and Django 2.0

[mblayman]

DJA should be tested against DRF 3.7.x and document that support.

[mblayman]

Django 2.0 support doesn't come unless DJA supports DRF 3.7. Since there have been a few requests for Django 2.0 support, I've updated the title so that new visitors will understand the connection.

[exit99]

Is this in progress?

[mblayman]

Yes, please see #394. TLDR: the PR is blocked by an upstream project for now.

[lucacorti]

Looks like django-polymorphic 2.0 was released a couple of days ago.

[mblayman]

Thanks. I've clearly not been watching it closely so I'll try to get my PR updated soon so we can work toward getting out a new DJA release.