DummyGenerator: Update application template

We do not have production group in generated dummy apps
and the syntax is outdated. This is what the Rails engine
generator does as well. So let's stick with the framework

Author: tvdeyen

core/lib/generators/spree/dummy/templates/rails/application.rb.tt

@@ -2,7 +2,7 @@ require File.expand_path('../boot', __FILE__)
 
 require 'rails/all'
 
-Bundler.require(*Rails.groups(assets: %w(development test)))
+Bundler.require(*Rails.groups)
 
 require '<%= lib_name %>'