Use PropTypes from module instead of React

Ref:
https://facebook.github.io/react/blog/2017/04/07/react-v15.5.0.html#migrating-from-react.proptypes

Author: joshbeckman

DateTime.js

@@ -1,13 +1,14 @@
 'use strict';
 
 var assign = require('object-assign'),
+        PropTypes = require('prop-types'),
         createClass = require('create-react-class'),
 	moment = require('moment'),
 	React = require('react'),
 	CalendarContainer = require('./src/CalendarContainer')
 ;
 
-var TYPES = React.PropTypes;
+var TYPES = PropTypes;
 var Datetime = createClass({
 	propTypes: {
 		// value: TYPES.object | TYPES.string,

package.json

@@ -78,6 +78,7 @@
   "dependencies": {
     "create-react-class": "^15.5.2",
     "object-assign": "^3.0.0",
+    "prop-types": "^15.5.7",
     "react-onclickoutside": "^5.9.0"
   },
   "pre-commit": [