Get edx-platform working with external completion lib, add integration tests.

openedx/tests/settings.py

@@ -75,7 +75,9 @@ INSTALLED_APPS = (
     'openedx.core.djangoapps.self_paced',
     'milestones',
     'celery_utils',
-    'lms.djangoapps.completion.apps.CompletionAppConfig',
+
+    # Django 1.11 demands to have imported models supported by installed apps.
+    'completion',
 )
 
 LMS_ROOT_URL = 'http://localhost:8000'