title: Django 1.8 to 1.9 upgrade: django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet
body: <p>I have a project currently working on Django 1.8. Since 1.9 just released, I thought I would update via <code>pip install django==1.9</code>. However, when running <code>python manage.py test -v 3</code>, I get this error:</p>
  
  <pre><code>Traceback (most recent call last):
    File "manage.py", line 11, in &lt;module&gt;
      execute_from_command_line(sys.argv)
    File "/home/user/Envs/intranet/lib/python2.7/site-packages/django/core/management/__init__.py", line 350, in execute_from_command_line
      utility.execute()
    File "/home/user/Envs/intranet/lib/python2.7/site-packages/django/core/management/__init__.py", line 324, in execute
      django.setup()
    File "/home/user/Envs/intranet/lib/python2.7/site-packages/django/__init__.py", line 18, in setup
      apps.populate(settings.INSTALLED_APPS)
    File "/home/user/Envs/intranet/lib/python2.7/site-packages/django/apps/registry.py", line 85, in populate
      app_config = AppConfig.create(entry)
    File "/home/user/Envs/intranet/lib/python2.7/site-packages/django/apps/config.py", line 90, in create
      module = import_module(entry)
    File "/usr/lib64/python2.7/importlib/__init__.py", line 37, in import_module
      __import__(name)
    File "/home/user/Envs/intranet/lib/python2.7/site-packages/reversion/__init__.py", line 11, in &lt;module&gt;
      from reversion.revisions import default_revision_manager, revision_context_manager, VersionAdapter
    File "/home/user/Envs/intranet/lib/python2.7/site-packages/reversion/revisions.py", line 18, in &lt;module&gt;
      from django.contrib.contenttypes.models import ContentType
    File "/home/user/Envs/intranet/lib/python2.7/site-packages/django/contrib/contenttypes/models.py", line 159, in &lt;module&gt;
      class ContentType(models.Model):
    File "/home/user/Envs/intranet/lib/python2.7/site-packages/django/db/models/base.py", line 94, in __new__
      app_config = apps.get_containing_app_config(module)
    File "/home/user/Envs/intranet/lib/python2.7/site-packages/django/apps/registry.py", line 239, in get_containing_app_config
      self.check_apps_ready()
    File "/home/user/Envs/intranet/lib/python2.7/site-packages/django/apps/registry.py", line 124, in check_apps_ready
      raise AppRegistryNotReady("Apps aren't loaded yet.")
  django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.
  </code></pre>
  
  <p>I am aware that INSTALLED_APPS changed from a tuple to a list.</p>
  
  <p>Anything else that I might have missed?</p>