Pass --no-clean to pip invocation during setup.py install

Without this option, calling pip while setup.py install is being run
results in pip deleting the build directory, causing the entire install
process to fail with difficult to debug errors about files being
missing. This fixes #179
Provide better feedback around automatic 3to2 installation
Merge pull request #219 from zoni/issue/179

Pass --no-clean to pip invocation during setup.py install