title: Webpack-dev-server serves old file content
body: <p>I have a very simple project:</p>
  
  <p><code>
  appdir
     +- app
        +- main.js
     +- build
        +- bundle.js
        +- index.html
     +- webpack.config.js
  </code></p>
  
  <p>The webpack.config.js:</p>
  
  <pre><code>var path=require("path");
  
  module.exports = {
      entry: path.resolve(__dirname, "./app/main.js"),
      output: {
          path: path.resolve(__dirname, "build"),
          filename: "bundle.js"
      }
  };
  </code></pre>
  
  <p>After I changs the <code>main.js</code>, webpack-dev-server seems like it detects the change and performs a rebundle the <code>bundle.js</code>, but the browser still recieve the old content of <code>main.js</code>.</p>
  
  <p>I start the server by executing <code>webpack-dev-server webpack.config.js</code></p>
  
  <p>Any ideas?</p>