Fix the invocation of get_pip.bash; raw.github.com now returns 301

A GET request for the URL:

  https://raw.github.com/mysociety/commonlib/master/bin/get_pip.bash

... currently returns a 301 Moved Permanently with Location:

  https://raw.githubusercontent.com/mysociety/commonlib/master/bin/get_pip.bash

However, curl won't follow redirects without -L, so zero bytes
were being piped to bash, pip wasn't being upgraded and the latest
setuptools wasn't being installed.