Replaces React.PropTypes references with prop-types

Author: cheton

src/ButtonGroup.jsx

@@ -1,5 +1,6 @@
 import classNames from 'classnames';
-import React, { PropTypes } from 'react';
+import PropTypes from 'prop-types';
+import React from 'react';
 import styles from './index.styl';
 
 const ButtonGroup = ({ className, btnSize, vertical, dropdownOpen, ...props }) => {