Fix warning when using react components like react-dropzone

[mairatma]

The react-dropzone component works well with metal-react, but it throws out the following warning:

Warning: Unknown props `elementClasses`, `events`, `visible` on <div> tag. Remove these props from the element. For details, see https://fb.me/react-unknown-prop
    in div (created by Dropzone)
    in Dropzone

All three of the props causing the warning are only being passed because all metal components have them, so we pass them over automatically via the bridge.