Add ActiveModel requirement to application.rb

Currently, ActiveModel is only loaded by ActiveRecord. If you skip ActiveRecord, ActiveModel will not be required (or even autoloaded) and including `ActiveModel::Model` into a plain Ruby class will raise `NameError`.

To reproduce this:

- create a new app with `rails new my_app -O`
- create a Ruby class that includes `ActiveModel::Model` in `app/models`
- load up a Rails console and try to do anything with the class :-)

Since ActionPack relies so heavily on the ActiveModel API, this should probably be considered a dependency of the app. Another possibility would be to make it a dependency of ActionController.