Add ASF license header to travis yaml file. (#301)

diff --git a/.travis.yml b/.travis.yml
index 4b9d47a..ee628f4 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,3 +1,6 @@
+# Licensed to the Apache Software Foundation (ASF) under one or more contributor
+# license agreements; and to You under the Apache License, Version 2.0.
+
 # A Travis CI configuration file.
 
 sudo: required