Fix the invocation of get-pip.py; raw.github.com now returns 301

A GET request for the URL:

  https://raw.github.com/pypa/pip/master/contrib/get-pip.py

... currently returns a 301 Moved Permanently with Location:

  https://raw.githubusercontent.com/pypa/pip/master/contrib/get-pip.py

However, curl won't follow redirects without -L, so zero bytes
were being piped to python and the new pip wasn't being installed.