[AIRFLOW-2779] Restore Copyright notice of GHE auth backend

At request of original author.

Author: ashb

NOTICE

@@ -13,6 +13,11 @@ is subject to the terms and conditions of their respective licenses.
 See the LICENSE file for a list of subcomponents and dependencies and
 their respective licenses.
 
+airflow.contrib.auth.backends.github_enterprise_auth:
+-----------------------------------------------------
+
+* Copyright 2015 Matthew Pelland (matt@pelland.io)
+
 hue:
 -----
 This product contains a modified portion of 'Hue' developed by Cloudera, Inc.