[FLINK-7494] [build] Add license header to '.travis.yml' file

This closes apache#4575

.travis.yml

@@ -1,3 +1,18 @@
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements. See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License. You may obtain a copy of the License at
+#
+# http:https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
 # s3 deployment based on http:https://about.travis-ci.org/blog/2012-12-18-travis-artifacts/
 
 # send to fully-virtualized infrastructure: https://docs.travis-ci.com/user/trusty-ci-environment/