ES6 classes + React 16 compatibility

This version supports React 15.5.0 and above.
Warnings from usage of createClass and React.PropTypes were cleaned.

Backwards compatibility

This version adds a peer dependency of React prop-types package.
I wrapped the require statement with a try-catch block, so it shouldn't break if you don't have prop-types.
However, the AMD tries to load prop-types when loading customScroll, so it will break without it.

In addition, without importing prop-types package, in all uses, you will not get prop types warnings for customScroll.