title: django constance: AttributeError: 'module' object has no attribute '_backend'
body: <p>I am upgrading site from django 1.4 to django 1.6.2, and the constance admin page goes wrong, which reports: </p>
  
  <pre><code>AttributeError: 'module' object has no attribute '_backend'
  </code></pre>
  
  <p>The error occurs at:</p>
  
  <pre><code>/python2.7/constance/admin.py in changelist_view, line 87
  </code></pre>
  
  <p>Which is: </p>
  
  <pre><code>    # First load a mapping between config name and default value
      if not self.has_change_permission(request, None):
          raise PermissionDenied
      default_initial = ((name, default)
          for name, (default, help_text) in settings.CONFIG.items())
      # Then update the mapping with actually values from the backend
      initial = dict(default_initial,
  87-&gt;      **dict(config._backend.mget(settings.CONFIG.keys())))
      form = ConstanceForm(initial=initial)
      if request.method == 'POST':
          form = ConstanceForm(request.POST)
          if form.is_valid():
              form.save()
              # In django 1.5 this can be replaced with self.message_user
  </code></pre>
  
  <p>I have upgraded constance, but doesn't work. Does anyone know the reason? Thanks!</p>