title: django-filer - AttributeError: 'Manager' object has no attribute '_inherited'
body: <p>I'm trying to install django-filer. But when I run <code>manage.py migrate</code> I get <strong>AttributeError: 'Manager' object has no attribute '_inherited'</strong>. I don't know what that means. (I started this project with Heroku starter template <a href="https://github.com/heroku/heroku-django-template" rel="nofollow noreferrer">https://github.com/heroku/heroku-django-template</a>)</p>
  
  <p>I initially thought that perhaps I forgot to install JPEG and ZLIB, but to my understanding they are default since Pillow 3.0.</p>
  
  <pre><code>$ pip freeze
  dj-database-url==0.4.1
  Django==1.10.4
  django-filer==1.2.5
  django-mptt==0.8.7
  django-polymorphic==1.0.2
  easy-thumbnails==2.3
  gunicorn==19.6.0
  psycopg2==2.6.2
  whitenoise==3.2
  Pillow==4.0.0
  </code></pre>
  
  <p>settings.py</p>
  
  <pre><code>INSTALLED_APPS = [
      ...
      'whitenoise.runserver_nostatic',
      'django.contrib.staticfiles',
      'easy_thumbnails',
      'filer',
      'mptt',
       ...
  ]
  </code></pre>