Fix South handling in requirements

South dependency is specified in setup.py, thus it must be removed
from requirements.txt where it was accidentally added.

requirements.txt

@@ -6,8 +6,7 @@ django>=1.4,<1.7
 # https://github.com/mariocesar/sorl-thumbnail/issues/255
 sorl-thumbnail>=11.12.1b
 
-# Migrations in Django
-South>=0.8.4
+# South (migrations in Django) is handled in setup.py
 
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