Add pyflakes to test-requires.

This ensures that pyflakes is installed in the venv and has the latest version.
Fixes a bug where old versions of pyflakes could not handle python 2.7
Merge pull request #72 from ameade/fix-pyflakes

Add pyflakes to test-requires.
Merge branch 'master' of git://github.com/rackspace-titan/qonos into retention