Make sure our 2 years old license headers are in line with

[dhermes]

Came up because of #2370. Related to #2371.

To paraphrase @rimey: The Apache license header given in our internal instructions differs from the one used in this project.

/cc @rimey @jgeewax

[jgeewax]

The correct one (according to our open-source friends) is:

Copyright 2016 Google Inc.

Licensed 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://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.