Support React v15.5+

[daisaru11]

I encountered an error:

Fatal Exception: RCTFatalException: Unhandled JS Exception: undefined is not an object (evaluating 'a.PropTypes.string')

This error seems to be same as #3 .

In React v15.5 and later, PropTypes needs to be imported from prop-types package (not react).
See:
https://reactjs.org/blog/2017/04/07/react-v15.5.0.html#migrating-from-reactproptypes