title: Laravel mix versioning doesn't remove old built files
body: <p>I am using Laravel 5.4 with mix to version my javascript and scss files.
  The problem is: it doesn't clean the previous built files and simply add a new one with a different file name i.e. <code>app.9d3e179e85922aad6ccf.js</code></p>
  
  <p>In a new project I started, I do not have this issue. Mix correctly replaces the old file with the latest built version.</p>
  
  <p>Is there somewhere where I can make the change?</p>