Remove unused imports

Fixes reactjs#92

Author: timdorr

lessons/13-server-rendering/README.md

@@ -87,7 +87,7 @@ into it.
 ```js
 // modules/routes.js
 import React from 'react'
-import { Router, Route, browserHistory, IndexRoute } from 'react-router'
+import { Route, IndexRoute } from 'react-router'
 import App from './App'
 import About from './About'
 import Repos from './Repos'