Fixing PropTypes warning messages (#2670)

* Fixing PropTypes warning message

React recently started warning on the upcoming deprecation of
React.PropTypes, the new approach is to use the `prop-types`
npm package instead.

* Fixing the tests

superset/assets/javascripts/profile/components/RecentActivity.jsx

@@ -1,11 +1,12 @@
 import React from 'react';
+import PropTypes from 'prop-types';
 import moment from 'moment';
 import $ from 'jquery';
 
 import TableLoader from './TableLoader';
 
 const propTypes = {
-  user: React.PropTypes.object,
+  user: PropTypes.object,
 };
 
 export default class RecentActivity extends React.PureComponent {