title: Celery Result backend. DisabledBackend object has no attribute _get_task_meta_for
body: <p>I have configured celery and the backend:</p>
  
  <pre><code>cleryapp = Celery(
      'tasks_app', brocker='amqp://guest@localhost//',
      backend='db+postgresql://guest@localhost:5432'
  )
  </code></pre>
  
  <p>'results' appears disabled when i start the worker, but I read on another question here that that's not the issue. </p>
  
  <p>The database is getting all the data correctly, but </p>
  
  <pre><code>result = AsyncResult(task_id)
  </code></pre>
  
  <p>raises </p>
  
  <pre><code>AttributeError: 'DisabledBackend' object has no attribute '_get_task_meta_for'
  </code></pre>