NUPIC#378 - pip is not part of the build process anymore, so do not auto clean python
Put back the python build_system clean command
Merge remote-tracking branch 'upstream/master' into encoders
Merge pull request #434 from keithcom/encoders

NUPIC#378 - pip is not part of the build process anymore, don't clean
Merge remote-tracking branch 'upstream/master' into graceful-missing-pytest-xdist