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
Gracefully handle missing pytest-xdist distribution.
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
Hard failure instead of graceful, from feedback provided by vitaly
Merge remote-tracking branch 'upstream/master' into graceful-missing-pytest-xdist
Merge pull request #442 from oxtopus/graceful-missing-pytest-xdist

Detect missing pytest-xdist module
Merge branch 'master' of github.com:numenta/nupic into issue_340