Update to React-Router v4

Currently, any `example` that uses react-router updated to v4 will break. I'm getting this error:

```
Unhandled rejection Error: Compilation completed with undefined assets. This likely means
react-static-webpack-plugin had trouble compiling one of the entry
points specified in options. To get more detail, try running again
but prefix your build command with:

DEBUG=react-static-webpack-plugin*

That will enable debug logging and output more detailed information.
at \node_modules\react-static-webpack-plugin\dist\index.js:189:15
```

Here are my files:

webpack.config.js
```javascript
module.exports = {
...
plugins: [
new ReactStaticPlugin({
routes: path.resolve(APP_DIR, 'routes.js'),
template: path.resolve(APP_DIR, 'template.js')
})
]
};
```

src/routes.js
```javascript
import { Route } from 'react-router-dom';

import Home from './routes/Home';

// Not sure about if this export is needed?
export const routes = ;

export default routes;
```

src/template.js
```javascript
import React from 'react';

const markup = ({ title = '', body }) => {
const titleSuffix = title === '' ? '' : ' - Hugmanrique';

{`${title}${titleSuffix}`}