Google's namespace "play-podcasts" is not recognized as valid

[stripecat]

Problem:
The validator does not recognize Google's namespace: http://www.google.com/schemas/play-podcasts/1.0. It will report is as an "unknown namespace".

Solution:
As it's valid and in use in many podcasts, it should be recognized as a valid namespace.