Explicitly require gem models

Fixes issue https://github.com/harrystech/seed_migration/issues/12, where rails `config.threadsafe!` turns off dependency loading ([see here](https://github.com/rails/rails/blob/02e4d7ffd4b96d13efd0c5ebba703606fc2e6414/railties/lib/rails/application/configuration.rb#L102)).