Update configuration.md

Fix the simplest configuration so it actually runs.

Author: MrP

content/concepts/configuration.md

@@ -30,6 +30,8 @@ The following examples below describe how webpack's configuration object can be
 **webpack.config.js**
 
 ```javascript
+var path = require('path');
+
 module.exports = {
   entry: './foo.js',
   output: {
@@ -180,4 +182,4 @@ export default (
     </plugins>
   </webpack>
 );
-```
+```