Update pyflakes

This updates the `pyflakes` submodule to its latest HEAD (the `0.6.1` release).

@kevinw the version in your `ftplugin/python/pyflakes` leaks variables from dict and set comprehensions, which hit me the other day, I had no idea where the errors were coming from, as both my system pyflakes and my fork were up to date, until I found that you have it pinned.

Thanks!