use prop-types package instead of deprecated React.PropTypes

[corime]

this adds another dependency but avoids deprecation warnings/errors once React.PropTypes are removed (see https://facebook.github.io/react/docs/typechecking-with-proptypes.html)

[danawoodman]

Thanks @coremi! Do you think this warrants a major version push? Will this break anything?

[corime]

Now it's a dependency instead of a peerDependency, and won't break anything. If it's ok for you? (for a major version push... I don't know, it just removes a deprecation warning, that only appears if a very new react version >=15.5 is used)

[danawoodman]

Thanks @coremi!