title: Uglifycss error when I do assetic:dump
body: <p>when I try to do the <code>php app/console assetic:dump --env=prod</code>  on Symfony I get the following exception:</p>
  
  <pre><code>  [Assetic\Exception\FilterException]
    An error occurred while running:
    '/usr/bin/nodejs' '/usr/local/bin/uglifycss' '/tmp/assetic_uglifycssv74yv7'
  
    Error Output:
  
    /usr/local/lib/node_modules/uglifycss/uglifycss:29
    const { defaultOptions, processString, processFiles } = require('./')
          ^
    SyntaxError: Unexpected token {
        at Module._compile (module.js:439:25)
        at Object.Module._extensions..js (module.js:474:10)
        at Module.load (module.js:356:32)
        at Function.Module._load (module.js:312:12)
        at Function.Module.runMain (module.js:497:10)
        at startup (node.js:119:16)
        at node.js:906:3
  </code></pre>
  
  <p>I use uglify-js and uglifycss. When it does the js it does it well, but when it reaches the css it gives me the previous error. It only happens when I do the command on the <code>--env=prod</code>. I don't know what else to do.</p>
  
  <p>Thanks!</p>