Include backend argument when instantiating Celery

Not including the backend argument can lead to AttributeError:
DisabledBackend object has no attribute for '_get_task_meta_for'.

See e.g.
http://stackoverflow.com/questions/23215311/celery-with-rabbitmq-attributeerror-disabledbackend-object-has-no-attribute.

At the same time, including the backend argument doesn't seem to harm anything else.

docs/patterns/celery.rst

@@ -36,7 +36,8 @@ This is all that is necessary to properly integrate Celery with Flask::
     from celery import Celery
 
     def make_celery(app):
-        celery = Celery(app.import_name, broker=app.config['CELERY_BROKER_URL'])
+        celery = Celery(app.import_name, backend=app.config['CELERY_BACKEND'],
+                        broker=app.config['CELERY_BROKER_URL'])
         celery.conf.update(app.config)
         TaskBase = celery.Task
         class ContextTask(TaskBase):