fix Python py verbose mode

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/26632712
[tdesai]PANTS_DEV=1 ./pants py --pex -v src/python/pants/bin:pants
Cleaning up...
Loading pants backends from source
Detected interpreter /usr/bin/python: CPython-2.6.7
Detected interpreter /System/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7: CPython-2.7.2
Detected interpreter /usr/local/bin/python2.7: CPython-2.7.5
/Users/tdesai/projects/pants src/python/pants/bin:pants
Build operating on targets: PythonBinary(BuildFileAddress(src/python/pants/bin/BUILD, pants))

Reviewed at https://rbcommons.com/s/twitter/r/445/