Consolidate Django and test requirements

[jpadilla]

Removes requirements-test.txt in favor of consolidating a minimum Django version and test requirements under requirements.txt.

Closes #2170.

[lovelydinosaur]

Yeah that seems neat & tidy to me.
We could do the whole '1.4.11+, 1.5.5+, 1.6, 1.7' Dance, but that seems a bit much for me - seems okay that some minor specifics are left implicit, and handled by the docs. This is afterall for development, not for installation purposed.

[xordoquy]

I'm probably missing a point, but wan't we simply put tox forward now that it's inline with travis ?

[jpadilla]

@xordoquy valid point as maybe we should be encouraging contributors to test using Tox. But I'm guessing not everyone has multiple python versions installed and using tox might be a bit "too much" than just using ./runtests.py locally and depending on Travis after they push.

[xordoquy]

You can still run an option that limits the builds to the existing Python. Also, they can choose their Python and Django versions they want to test against.

[lovelydinosaur]

Personally I only run tox occasionally, but use a plain runtests.py for most things. I think it does make sense for us to continue to include a plain requirements file.