Flake8

In my quest to update django-moderation so that it can be used with Python 3, I have come across an issue with pyflakes. Notably it doesn't work with Python 3 and that it is a few years old.
However, Flake8 supersedes both pyflakes and pep8, keeping the required functionality.

With the move to Flake8, I have fixed the errors that it has picked up (mainly unused imports).

I also came across a problem with some of the required packages when running on Python 2.6. A package depends on ipython, but the latest version of ipython doesn't support Python 2.6, so I've added a condition to setup.py that if the interpreter version is less than 2.7, then the suitable version of ipython should be installed.

As you can see on Travis-CI, the tests are passing now: https://travis-ci.org/Moustacha/django-moderation/builds/25391224