Synchronize license for silk icons
Synchronize license for silk icons

According to the homepage [1] silk icons are licensed under
a Creative Commons Attribution 2.5 License, but can also be used
under Creative Commons Attribution 3.0 License with conditions.

Update the license file accordingly.

[1] http://famfamfam.com/lab/icons/silk/

Change-Id: I116f774732fa64162c142eaad1ef59e3a1ec462a

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