Make compatible with Django 1.4

[flashingpumpkin]

Currently failing with:

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ERROR: test_cycle (riak_sessions.tests.RiakSessionTest)
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Traceback (most recent call last):
  File "/home/alen/.virtualenvs/django-riak-sessions/local/lib/python2.7/site-packages/django/contrib/sessions/tests.py", line 161, in test_cycle
    self.session.cycle_key()
  File "/home/alen/.virtualenvs/django-riak-sessions/local/lib/python2.7/site-packages/django/contrib/sessions/backends/base.py", line 241, in cycle_key
    self.create()
  File "/home/alen/projects/django-riak-sessions/riak_sessions/backends/riak.py", line 31, in create
    self.session_key = self._get_new_session_key()
AttributeError: can't set attribute

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ERROR: test_flush (riak_sessions.tests.RiakSessionTest)
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Traceback (most recent call last):
  File "/home/alen/.virtualenvs/django-riak-sessions/local/lib/python2.7/site-packages/django/contrib/sessions/tests.py", line 150, in test_flush
    self.session.flush()
  File "/home/alen/.virtualenvs/django-riak-sessions/local/lib/python2.7/site-packages/django/contrib/sessions/backends/base.py", line 233, in flush
    self.create()
  File "/home/alen/projects/django-riak-sessions/riak_sessions/backends/riak.py", line 31, in create
    self.session_key = self._get_new_session_key()
AttributeError: can't set attribute

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FAIL: test_invalid_key (riak_sessions.tests.RiakSessionTest)
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Traceback (most recent call last):
  File "/home/alen/.virtualenvs/django-riak-sessions/local/lib/python2.7/site-packages/django/contrib/sessions/tests.py", line 173, in test_invalid_key
    self.fail("The session object did not save properly.  Middleware may be saving cache items without namespaces.")
AssertionError: The session object did not save properly.  Middleware may be saving cache items without namespaces.

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