Use prop-types package

Author: Patrick Burtchaell

src/notificationStack.js

@@ -1,5 +1,6 @@
 /* eslint-disable react/jsx-no-bind */
-import React, { PropTypes } from 'react';
+import React from 'react';
+import PropTypes from 'prop-types';
 import StackedNotification from './stackedNotification';
 
 function defaultStyleFactory(index, style) {