Docs: Just list required stuff in installation

There is no need to talk about Django because the procedure listed below
already installs the right Django version by using pip. Also the highlight
of the supported Django versions belongs to the release notes.

docs/server/installation.rst

@@ -9,7 +9,6 @@ requirements in a virtual environment.
 Before proceeding, consider installing these first:
 
 - At least Python 2.6
-- At least Django 1.5.5 (Won't work yet with Django 1.6 or newer)
 - `python-pip <http://www.pip-installer.org/>`_
 
 If you only want to have a sneak peek of Pootle, the default configuration and