I also use Ubuntu 12.04LTS with nginx and ngx_pagespeed
You can follow this link to compile nginx with ngx_pagespeed: https://github.com/pagespeed/ngx_pagespeed#how-to-build You can increase your score without requiring to modify your code.
If you want to get a perfect PageSpeed score, this article might help you: http://www.appneta.com/blog/bootstrap-pagespeed . https://github.com/danriti/bootstrap-pagespeed