Drop support for old versions

[LilyFoote]

We currently support:

• python
  • 2.7
  • 3.3
  • 3.4
• django
  • 1.6
  • 1.7
  • 1.8
• DRF
  • 3.1
  • 3.2

I think we should consider dropping support for the bold versions.

[LilyFoote]

@meshy

[meshy]

What incompatibilities make supporting DRF 3.1 and python 3.3 harder?

[LilyFoote]

It's not just about incompatibilities. It also leads to a lot of test runs on travis - currently 11 (4 django * 3 python - 1). We should really include DRF 3.1 as well which would mean 22 or so.