Update README with 8th lesson on refactoring from ES5 to ES6

The eighth lesson goes over how to refactor the React app from
ES5 to ES6.

Update babel-register (server) and babel-loader (webpack) to use
`babel-preset-es2015` to compile ES6 to ES5.

Refactor <Layout> to a class component and <Index> and <About>
to stateless functional components.

YouTube:
- Title: Refactor a React app from ES5 to ES6
- Link: https://www.youtube.com/watch?v=TD8Z3hw_M_o

Author: remarkablemark

README.md

@@ -42,23 +42,23 @@ $ npm install
 $ npm start
 ```
 
-[4. Server-side routing with react-router](https://b.remarkabl.org/2dCiZ06):
+[4. Server-side routing with react-router@3](https://b.remarkabl.org/2dCiZ06):
 
 ```sh
 $ git checkout v4
 $ npm install
 $ npm start
 ```
 
-[5. Client-side routing with react-router (Part 1)](https://b.remarkabl.org/2faLHsH):
+[5. Client-side routing with react-router@3 (Part 1)](https://b.remarkabl.org/2faLHsH):
 
 ```sh
 $ git checkout v5
 $ npm install
 $ npm start
 ```
 
-[6. Client-side routing with react-router (Part 2)](https://b.remarkabl.org/2fOOWn7):
+[6. Client-side routing with react-router@3 (Part 2)](https://b.remarkabl.org/2fOOWn7):
 
 ```sh
 $ git checkout v6
@@ -74,6 +74,14 @@ $ npm install
 $ npm start
 ```
 
+[8. Refactor a React app from ES5 to ES6](https://b.remarkabl.org/2q5jl8M):
+
+```sh
+$ git checkout v8
+$ npm install
+$ npm start
+```
+
 ## License
 
 [MIT](https://github.com/remarkablemark/universal-react-tutorial/blob/master/LICENSE)