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/explorev2/components/EmbedCodeButton.jsx

@@ -1,4 +1,5 @@
-import React, { PropTypes } from 'react';
+import React from 'react';
+import PropTypes from 'prop-types';
 import { Popover, OverlayTrigger } from 'react-bootstrap';
 import CopyToClipboard from './../../components/CopyToClipboard';