title: Relative URLs not being changed correctly by Django compressor
body: <p>For exmaple, in my SCSS file,</p>
  
  <pre><code>url('../../fonts/Museo_Slab_500-webfont.eot');
  </code></pre>
  
  <p>turns into</p>
  
  <pre><code>url('https://fonts/Museo_Slab_500-webfont.eot');
  </code></pre>
  
  <p>which obviously does not work.</p>
  
  <p>I have in my settings.py</p>
  
  <pre><code>DEFAULT_FILE_STORAGE = 'storages.backends.s3boto.S3BotoStorage'
  AWS_ACCESS_KEY_ID = os.environ['AWS_ACCESS_ID']
  AWS_SECRET_ACCESS_KEY = os.environ['AWS_ACCESS_KEY']
  AWS_STORAGE_BUCKET_NAME = 'digital_time_capsules'
  STATICFILES_STORAGE = 'storages.backends.s3boto.S3BotoStorage'
  AWS_QUERYSTRING_AUTH = False
  COMPRESS_OFFLINE = True
  COMPRESS_STORAGE = STATICFILES_STORAGE
  COMPRESS_URL = "https://digital_time_capsules.s3.amazonaws.com/"
  STATIC_URL = COMPRESS_URL
  </code></pre>
  
  <p>I tried several times to run compress, and restart the server, and whatnot. It worked once, yesterday, and I got the full path to S3. But now it doesn't, and my custom resources don't load.</p>
  
  <p>Bootstrap's URLs seem to always work, though. It did successfully generate</p>
  
  <pre><code>image:url('https://digital_time_capsules.s3.amazonaws.com/img/glyphicons-halflings-white.png')
  </code></pre>
  
  <p>Any ideas?</p>