Increase the JS Cache control header time

[codedance]

At the moment caching is set at 30-days. In affect this is "forever" and any dev must use cache busting methods (which silken supports) to ensure clients get a new version. I suggest we target 1 year which is the max outlined in the RFC: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.21