Update pep8 to 1.5.7

This is just to keep us on the upgrade treadmill. ([Changelog](https://pypi.python.org/pypi/pep8#changelog))
@cpennington I can't figure out quickly whether the `InheritanceKeyValueStore` fixes this bug or merely moves it. The problem is that delete or set of a field should get rid of any cached values for inheriting that field in the subtree below the xblock. I still see a lot of code in split having to do w/ inheritance but from what I can tell, it's wasted work that doesn't get used. Of course, the jira number is from the old jira not the one we're using now.

Anyway, I think we need to get rid of code in split which is mooted by the InheritanceKVS and see if the InheritanceKVS has this bug.