Revert "upgrade gunicorn 19.0 and make upgrade"

This reverts commit f6c215ae5b.

lms/djangoapps/instructor_task/api_helper.py

@@ -135,13 +135,11 @@ def _get_xmodule_instance_args(request, task_id):
     permit old-style xqueue callbacks directly to the appropriate module in the LMS.
     The `task_id` is also passed to the tracking log function.
     """
-    host_header = 'HTTP_HOST' if request is not None and 'HTTP_HOST' in request.META else 'SERVER_NAME'
-
     request_info = {'username': request.user.username,
                     'user_id': request.user.id,
                     'ip': request.META['REMOTE_ADDR'],
                     'agent': request.META.get('HTTP_USER_AGENT', '').decode('latin1'),
-                    'host': request.META[host_header],
+                    'host': request.META['SERVER_NAME'],
                     }
 
     xmodule_instance_args = {'xqueue_callback_url_prefix': get_xqueue_callback_url_prefix(request),