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.

@sarina and @stvstnfrd 

I think the only point of contention is whether we should have developer requirements mixed in with the strict source requirements. I think that:
1. having multiple requirements files is a drag
2. pep8 is a pretty special thing that most developers immediately recognize
3. having pep8 installed (even in a dedicated docker container) won't hurt anything