Update pep8 to 1.5.7

This is just to keep us on the upgrade treadmill. ([Changelog](https://pypi.python.org/pypi/pep8#changelog))
@sarina I'd rather do the max-line-length thing in a separate pull request, since I suspect it will reveal a few new violations that will then need to be fixed to get back down to zero. I'll add comments explaining E265 and W602, though.