Cache 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, I think we should rely on the gem.

We are using this in staging and production on a current thoughtbot project.

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