changed propTypes to use npm package prop-types

[NicholasLYang]

Just updated propTypes to use the separate npm package prop-types. propTypes in react will be deprecated with v16.0.0

[rob2d]

Thanks for taking care of that Nick. Since the prop-types aren't actually used, we can actually also remove the import (which would cut down a bit of overhead for the base template). Will make note of that. Still debating whether to include prop-types or not because there is a minority of React devs that opt either not to use them now that stateless components describe props better, or to go with typescript.