Run tests on multiple Django versions

[nikolas]

A recent code change broke behavior on Django 1.8 (#525). Django 1.8 is an LTS release, and lettuce should continue to support it as long as the django team does. Most django libraries run their test suite on multiple versions of django, and lettuce should follow suit to prevent this from happening.

Many people use tox for this, but I've gotten by with just configuring travis for multiple django versions, e.g.: https://github.com/ccnmtl/django-pagetree/blob/master/.travis.yml

I just wanted to note this here so I wouldn't forget it. I want to set this up at some point, but if someone else wants to jump in that'd be great.

[michelts]

Fixed in #552

But I choose create the environments in tox instead of travis, so we have one unique way to run tests. Let me know if you prefer the environments to be in travis.