Fix listenBeforeLeavingRoute error

I had to add 'react/addons' as external due to a `Cannot read property 'listenBeforeLeavingRoute' of undefined` error. Got it from here remix-run/react-router#1093 (comment)

Author: luisherranz

lessons/13-server-rendering/README.md

@@ -37,7 +37,7 @@ module.exports = {
 
   // keep node_module paths out of the bundle
   externals: fs.readdirSync(path.resolve(__dirname, 'node_modules')).concat([
-    'react-dom/server'
+    'react-dom/server', 'react/addons',
   ]).reduce(function (ext, mod) {
     ext[mod] = 'commonjs ' + mod
     return ext