Merge in fixes for cache control

This, combined with the ops changes in mozilla-services/puppet-config#320 on the nginx configuration improves the performance *and* reduces the resource usage on the server. 

This brings the [pingdom website speed test](http://tools.pingdom.com/fpt/#!/b8kDnt/https://bwong-content-elb-1uzfjoov8nu7g-358530316.us-west-2.elb.amazonaws.com/signup) up to 89/100. The last mile would be improvements to the JS/CSS loading that we can do on the server side.