Add testing on newer versions of DRF and Django

[kevin-brown]

This library hasn't changed much and still appears to be supported in newer versions of Django REST framework, but it's difficult to verify because the test matrix stops at DRF 3.1. This updates the test matrix for newer versions of DRF.

Changes:

• Added testing for DRF versions up to DRF 3.8
• Added testing for Django versions up to Django 2.0
• Reorganized testing matrix to only test Python and Django versions that are supported by the DRF versions

This maintains testing and support for the existing DRF and Django combinations, since no code changes were necessary to support newer combinations.

[jpadilla]

@kevin-brown thank you so much for this ✨