Fixed gemfile format for admin and email_dev recipes.

I didn't realize there was a difference between the `gem` and `add_gem` helpers. Changed to fix the formatting of the admin and email_dev recipe gems in the Gemfile.

Should probably also update the example recipe in [the "Recipe Structure" section of the wiki](https://github.com/RailsApps/rails_apps_composer/wiki/tutorial-rails-apps-composer#wiki-anatomy-of-a-recipe) to use the `add_gem` helper.