Serve static assets, log to STDOUT in staging/prod

The rails_12factor gem currently does two things in staging and
production environments:

* config.serve_static_assets = true
* Logger.new(STDOUT)

Rather than manage the intricacies of how these settings will change in
future versions of Rails, we should rely on the gem.

https://devcenter.heroku.com/articles/rails-integration-gems
https://github.com/heroku/rails_12factor
https://github.com/heroku/rails_serve_static_assets
https://github.com/heroku/rails_stdout_logging

templates/Gemfile_clean

@@ -44,4 +44,5 @@ end
 
 group :staging, :production do
   gem 'newrelic_rpm', '>= 3.5.7'
+  gem 'rails_12factor'
 end