Add a python prefix for `diff-quality` task

@wedaly `diff-quality` is reporting extra violations because we don't set the python path properly. `rake pylint` sets the path, and thus is more accurate in reporting violations. I'd like to fix this in the same manner as rake pylint - from further up in `quality.rake` - so that the two reports are analogous. 

However... this doesn't work, and I can't figure out why it's unhappy. Rake seems to be failing on the `#{pythonpath_prefix}` bit, as seen here: https://gist.github.com/sarina/5c98dd3109c6d3f6cac5

Any ideas?