Switching to DS.ActiveModelAdapter so underscore to camel works by default.

From viewing https://github.com/emberjs/data/issues/1180 and https://github.com/emberjs/data/pull/1206, it appears that the direction moving forward should be to use the DS.ActiveModelAdapter in this project instead of DS.RESTAdapter.  If this is not the case it would be nice to get `ActiveModel::Serializers` setup to use camelCasing by default in this project instead.  Currently, without taking either action, things just don't work correctly.  This should make it work by default.