React v15.5 update causes breaking changes

`PropTypes` must now be imported from the `prop-types` package, as mentioned in the React v15.5 announcement: https://reactjs.org/blog/2017/04/07/react-v15.5.0.html

From the announcement:
>Prop types are a feature for runtime validation of props during development. We’ve extracted the built-in prop types to a separate package to reflect the fact that not everybody uses them.
>In 15.5, instead of accessing PropTypes from the main React object, install the prop-types package and import them from there:

`React.PropTypes` is used here: https://github.com/Autodesk/molecule-3d-for-react/blob/ea30999599755f916a1db1d321b198e21348d1a6/src/components/molecule_3d.jsx#L53