Document that we require South 0.7.5

Earlier versions seem to fire post_migrate signal even when the given
migration is not complete, what leads to executing current code on not
yet fully migrated database, what obviously fails.

Fixes #482
Signed-off-by: Michal Čihař <michal@cihar.com>

docs/admin/install.rst

@@ -26,7 +26,7 @@ PIL or Pillow library
     http://python-imaging.github.io/
 lxml (>= 3.1.0)
     http://lxml.de/
-south
+South (>= 0.7.5)
     http://south.aeracode.org/
 libravatar (optional for federated avatar support)
     https://pypi.python.org/pypi/pyLibravatar

requirements.txt

 Django>=1.5
 GitPython>=0.3.2.RC1
-South>=0.7
+South>=0.7.5
 Whoosh>=2.5.2
 Translate-Toolkit>=1.10.0
 lxml>=3.1.0