Lock the version of virtualenv.

The latest version of virtualenv install pip 1.5 which uses distlib which expects
a home directory where it can create a '.distlib' directory.

See: https://bitbucket.org/pypa/distlib/issue/39/using-distlib-with-pip-fails-when-usre-has
Explicitly set the npm registry.
Use npm config instead of editing rc files directly.
Merge pull request #603 from edx/feanil/lock_virtualenv

Feanil/lock virtualenv