Do not automatically compression assets.

We are serving compressed files to all clients whether they request (or support) compression or not. This is certainly not RFC compliant, and I don't see a way to make S3 take care of this automatically for us.

Background: 

https://github.com/emberjs/ember-rails/issues/243
https://github.com/emberjs/ember-dev/commit/d0153d3c2265bac9c42663ff3b1a3887cf00ba74