Relax dependency restrictions for South and django-compressor

Oscar 0.7 works ok with later versions of these two packages and so we
open up the restrictions in setup.py to avoid forced downgrades for
Oscar projects.

Fixes #1477
Bump version to 0.7.3 and update release notes
Merge branch 'releases/0.7'

Backporting the changes for 0.7.3.

Conflicts:
	docs/source/releases/index.rst
	oscar/__init__.py
	setup.py