Remove a hard dependency on ActiveModelSerializers

Based on my own experience and a number of blog posts, it looks like people are using various solutions (Rails-API, etc) to build Ember-compatible APIs in Rails, not just ActiveModelSerializers.  Nothing in `ember-rails` requires to have the AMS dependency, so I'd like to raise for consideration removing this hard dependency.

It's a great convention when starting a new app, but I think many developers will try to add `ember-rails` to an existing Rails application, and if their experience is anything like mine, it can mess up existing API implementations.  In my example, I had my own responder that was overridden by AMS.  I found no way to turn it off.

To solve this problem, I forked the gem without the AMS dependency as `ember-rails-lite`, but I figured I'll submit this for general consideration.