distribute was merged into setuptools as of setuptools 0.7

The bin/prepare_environment.bash script ensures that we have the most
recent setuptools installed, so this is unnecessary.  distribute 0.7.3
will fail to install many of the packages depended on by Pombola, so
it's just confusing to have it specified in here.
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.