Adding css to format_listener rules

Adding css to priorities array to use the css files in the demo content. Without you see the plain html page.

Author: jschmahl

app/config/config.yml

@@ -343,4 +343,4 @@ fos_rest:
         json: true
     format_listener:
         rules:
-            - { path: ^/, priorities: [ html, json, xml ], fallback_format: html, prefer_extension: false }
+            - { path: ^/, priorities: [ html, json, xml, css ], fallback_format: html, prefer_extension: false }