Updated gatsby-config to reflect minor plug-in changes

gatsby-config.js

@@ -63,6 +63,10 @@ module.exports = {
             options: {
               defaultText: 'Try it on CodePen',
               directory: `${__dirname}/examples/`,
+              externals: [
+                `//unpkg.com/react/umd/react.development.js`,
+                `//unpkg.com/react-dom/umd/react-dom.development.js`,
+              ],
               redirectTemplate: `${__dirname}/src/templates/codepen-example.js`,
               target: '_blank',
             },