Replaced deprecated React#getDOMNode
Replaced deprecated React#getDOMNode

Replaced deprecated React#getDOMNode with React.findDOMNode which caused an error
``Uncaught TypeError: Cannot read property 'getBoundingClientRect' of undefined``

Error occurs when usingReact via ES6 class syntax.

Related note from [React documentation](https://facebook.github.io/react/blog/2015/03/10/react-v0.13.html)
> Added new top-level API React.findDOMNode(component), which should be used in place of component.getDOMNode(). The base class for ES6-based components will not have getDOMNode. This change will enable some more patterns moving forward.
vanderlin/react-scroll,daviferreira/react-scroll,ntdb/react-scroll,dandelany/react-scroll,slashtu/react-scroll,fisshy/react-scroll,synapsestudios/react-scroll