React 16 support

[Graham42]

I'm attempting to update react-transition-group to react 16 but there's several tests failing with errors from within this library with "TypeError: _utils.getInstanceFromNode is not a function"

 FAIL  test/CSSTransition-test.js
  ● CSSTransition › should flush new props to the DOM before initiating a transition

    TypeError: _utils.getInstanceFromNode is not a function

      at normalizeSubject (node_modules/bill/node.js:57:57)
      at createNode (node_modules/bill/node.js:118:13)
      at NodeCollection (node_modules/teaspoon/index.js:49:44)
      at Object.<anonymous> (test/CSSTransition-test.js:17:28)
          at Promise (<anonymous>)
      at Promise.resolve.then.el (node_modules/p-map/index.js:42:16)
          at <anonymous>

For reference my branch is here: https://github.com/Graham42/react-transition-group/tree/react-16