title: Webpack - extract-text-webpack-plugin Cannot find module
body: <p><strong>webpack.config.js</strong></p>
  
  <pre><code>var ExtractTextPlugin = require("extract-text-webpack-plugin");
  </code></pre>
  
  <p>I immediately receive this error if I just implement the plugin:</p>
  
  <pre><code>module.js:339
      throw err;
      ^
  
  Error: Cannot find module 'webpack/lib/ConcatSource'
      at Function.Module._resolveFilename (module.js:337:15)
      at Function.Module._load (module.js:287:25)
      at Module.require (module.js:366:17)
      at require (module.js:385:17)
      at Object.&lt;anonymous&gt; (/Users/lucamormile/Documents/Lavori/Webapps/React/webpack_test/node_modules/extract-text-webpack-plugin/index.js:5:20)
      at Module._compile (module.js:425:26)
      at Object.Module._extensions..js (module.js:432:10)
      at Module.load (module.js:356:32)
      at Function.Module._load (module.js:311:12)
      at Module.require (module.js:366:17)
  </code></pre>
  
  <p>What did i forgot?</p>