PEP8: add in flake8 checking
don't actually fail this till #11928 is clean.

@jreback so this gets run before the actual build right? Is that where we want it? Or would we rather have the tests run, get the feedback about the tests, and potentially fail afterwards?
 yeah could run it after
 better? https://travis-ci.org/jreback/pandas/jobs/99880071

its at the end in its own section. It will pass for now (but show you the output)
