Add support for DRF 3.9

[orn688]

Django REST Framework 3.9.0 was released on October 18th, but DRF-GIS only supports up to 3.8.x. I can take a stab at checking compatibility if that's an unknown.

[imomaliev]

Already have PRs for this #173 and #175

[imomaliev]

@auvipy @nemesisdesign are we going to make a release with updated drf version or we are waiting for some additional PRs to be merged?

[nemesifier]

thanks for reminding @imomaliev, will try to that by the end of this week

[imomaliev]

@nemesisdesign I suggest updating docs that this project doesn't support 2.7 anymore, because I removed failing travis entry. I will provide code cleanup from python 2 code after release for drf 3.9

[nemesifier]

@imomaliev done