Incompatible with React 15.5+

First, great library and I'm surprised there aren't more codebases using it.

Second, react-ml relies on React.PropTypes which have [been migrated to the separate module](https://facebook.github.io/react/blog/2017/04/07/react-v15.5.0.html#migrating-from-react.proptypes) 'prop-types'.

Would it be possible to adjust the import statements to fix this issue? I tried doing it myself but ran into compiler issues on my own machine.