DraggableResizableBox Warning

[kennyhk]

Please update the DraggableResizableBox for 2 warning :

Warning: Accessing PropTypes via the main React package is deprecated. Use the prop-types package from npm instead.

Warning: DraggableResizableBox: React.createClass is deprecated and will be removed in version 16. Use plain JavaScript classes instead. If you're not yet ready to migrate, create-react-class is available on npm as a drop-in replacement.

Thank you very much!

[mkozhukharenko]

now it is not just a warning - it leads to an error.

I've submitted a PR #23