[AIRFLOW-2815] Use correct copyright period

"onwards" is not specific enough
2018-07-28 13:10:43 +02:00 · 2018-07-28 13:10:43 +02:00 · 4f562ba8c7
--- a/2
+++ b/2
@ -1,5 +1,5 @@
 Apache Airflow
-Copyright 2016 and onwards The Apache Software Foundation
+Copyright 2016-2018 The Apache Software Foundation


 This product includes software developed at The Apache Software