title: Error when trying to compress static files when using django-storages and django-compressor together
body: <p>I have setup an AWS S3 bucket in order to transfer my static files in a remote CDN using the application django-storages, 
  everything worked fine until I tried to compress my static files before uploading to S3 using django_compressor.</p>
  
  <p>I have setup all the variables according django_compressor documentation for django-storages (<a href="https://django_compressor.readthedocs.org/en/latest/remote-storages/index.html" rel="nofollow">https://django_compressor.readthedocs.org/en/latest/remote-storages/index.html</a>)</p>
  
  <p>I uploaded all the files in S3 using 'manage.py collectstatic' then:</p>
  
  <p><strong>When I do 'manage.py compress' I get this error:</strong></p>
  
  <pre><code>CommandError: An error occured during rendering ../templates/base.html: 'https://my_bucket.s3.amazonaws.com/css/bootstrap.2.3.1.css' isn't accessible via COMPRESS_URL ('https://my_bucket.s3-external-3.amazonaws.com/') and can't be compressed
  </code></pre>
  
  <p>What's wrong with my setup?</p>
  
  <p><strong>Here is my settings.py configuration for django-storages and django_compressor:</strong></p>
  
  <pre><code>COMPRESS_URL = 'https://mybucket_name.s3-external-3.amazonaws.com/'
  STATIC_URL = COMPRESS_URL
  DEFAULT_FILE_STORAGE = 'my_project.boto_custom.CachedS3BotoStorage'
  
  AWS_ACCESS_KEY_ID = 'XXX'
  AWS_SECRET_ACCESS_KEY = 'XXX'
  AWS_STORAGE_BUCKET_NAME = 'mybucket_name'
  
  COMPRESS_ROOT = STATIC_ROOT
  
  COMPRESS_STORAGE = 'my_project.boto_custom.CachedS3BotoStorage'
  STATICFILES_STORAGE = 'my_project.boto_custom.CachedS3BotoStorage'
  
  COMPRESS_OFFLINE = True
  </code></pre>
  
  <p>Thanks for your help</p>