Reduce asset-related logging

This turns off log messages from Sprockets, eg:

   Served asset /jquery.ui.effect.js - 304 Not Modified (4ms)

Unfortunately it doesn't stop the messages from serving the request:

   Started GET "/assets/jquery.ui.effect.js?body=1" for 127.0.0.1 at 2013-08-21 16:28:24 +0000