title: assetic:dump fails on "less" filter
body: <p>When I try and dump my assetic managed content for a Symphony2 app, I get the following error:</p>
  
  <pre><code>$ php app/console assetic:dump -e prod
  Dumping all prod assets.
  Debug mode is off.
  
    [InvalidArgumentException]
    There is no "less" filter.
  </code></pre>
  
  <p>However, as far as I can tell I don't use any assets that require the less filter, certainly a grep of all my twig templates doesn't turn anything up.</p>
  
  <p>One of the dependencies I've installed via composer (Twitter's bootstrap) has some .less templates, but I don't reference them in my twig templates, I just point it at the css version. Will assetic still try and dump them? How can I tell it not to?</p>
  
  <p>For reference this is how I include the css in my template</p>
  
  <pre><code>{% stylesheets filter="cssrewrite"
    '../vendor/jquery-ui-css/jquery-ui-css/*css'
    '../vendor/twitter/bootstrap/docs/assets/css/bootstrap*.css'
  %}
  &lt;link rel="stylesheet" type="text/css" href="{{ asset_url }}"&gt;
  {% endstylesheets %}
  </code></pre>