Update installation.rst

Clarify documentation and be specific about the supported Django versions.
Merge pull request #533 from wimfeijen/patch-1

Update installation.rst
Rewrap a few lines
Merge branch 'master' into next, be bold and declare that we are compatible with Django 1.7.

* master:
  Rewrap a few lines
  Update installation.rst

Conflicts:
	docs/installation.rst