Summary:
react@16 (a peerDependency) did away with the PropTypes export in favor of the prop-types module.
This updates all of the remaining references to `React.PropTypes`.
Closes https://github.com/facebook/react-native/pull/14641
Differential Revision: D5287167
Pulled By: javache
fbshipit-source-id: a917e29aa0e5470260568995dfe97f5528ec265e