Summary:
Switch web player cdn to npmcdn per discussion with lacker. This will make the url agnostic to who owns the git repo.
Closes https://github.com/facebook/react-native/pull/8426
Differential Revision: D3488755
Pulled By: lacker
fbshipit-source-id: b54dd4428a48c8a5a15b0b38ee0564d119916f9b