Fix the py command and add py in the ci script

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/26624784

Local testing

tw-172-25-20-142 pants]$ PANTS_DEV=1 ./pants py --pex src/python/pants:pants
*** Running pants in dev mode from /Users/tdesai/projects/pants/src/python/pants/bin/pants_exe.py ***
Loading pants backends from source
/Users/tdesai/projects/pants src/python/pants:pants
WARNING: Target has multiple python_binary targets!
Wrote /Users/tdesai/projects/pants/dist/src.python.pants.pants.pex

Bugs closed: 179

Reviewed at https://rbcommons.com/s/twitter/r/444/