Install django via pip instead in order to get version 1.5.4

This module currently installed some components via the PyPI, and some via apt.  Unfortunately, 14.04 includes django 1.6.1 which is incompatible with the version of graphite-web pulled in via pip.

This change ensures that the django components are pulled in from the former and that a compatible version is installed.

Further background / reading here: http://osdir.com/ml/general/2013-11/msg29595.html