drf-tracking django 2 ready

[triat]

I saw more and more discussion about updating to django2 and none which was complete I think.

So I tried also to summarize changes that I saw in different other PR and I made one.

Here what was done:

• Changed the tox file with new Django versions
• Removed everything that was not supported anymore
• Adapted the code and tests to work with the new changes

This update might break old versions of DRF / Django / python but as they are not supported anymore, I took the right to keep only the one recommended right now.

I'm trying to cover #85 #115

[aicioara]

@avelis Are you planning a release with these changes?