`rails generate` runs correctly under Rails 3.0.x
`rails generate` runs correctly under Rails 3.0.x

* Rails 3.0.x does not yield `app` to `generator` blocks in railties.
  Picks a hopefully reasonable default `Rails.application` in that case.
* Fixes #71

adparlor/active_model_serializers,ahey/active_model_serializers_latest,younited/active_model_serializers,getoutreach/active_model_serializers,onehub/active_model_serializers