recommend using the SPDX license identifier

as it's easier for tools to recognize.

It's also recommended in the bower.json spec: https://github.com/bower/bower.json-spec#license

SPDX license list: http://spdx.org/licenses/

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Should also be updated here: https://github.com/GoogleWebComponents/google-youtube/blob/v0.0.4/bower.json#L16

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@addyosmani