Require pep8 now that we have a test dependency

* Since ccbdd82f3b420a535fb78f78a3695d584f784296, travis builds require
  pep8 to pass for the build to pass at all. This places the onus on the
  developer not to increase pep8 violations.
* Consequently, there is a practical dependency on having pep8 to do
  work on this codebase, even if there is no strict technical
  requirement for it.
* Install pep8 by default so developers can have more fun.
Merge pull request #27 from edx/jrbl/require_pep8

Require pep8 now that we have a test dependency