ensure that a compatible version of pip is installed
ensure that a compatible version of pip is installed

This diff pegs the version to 1.5.6, it seems that 6.0 changed the api and doesn't work, and 1.4 doesn't contain pip._vendor.
chriskuehl/pip-faster,Yelp/pip-faster,Yelp/venv-update,Yelp/venv-update,Yelp/venv-update,chriskuehl/pip-faster,Yelp/pip-faster,Yelp/pip-faster