If you run into problems with assets located under the public directory of your rails application, please have a look at your production.rb located at <your_rails_home>/config/environments/production.rb.
Search for the line containing
config.serve_static_assets = false
and replace it with
config.serve_static_assets = true
This setting enables Rails to serve the static assets instead of letting Apache or Nginx do the job.
Run 'cf push' to redeploy your application with the new environment setting.
Your application should be able to serve the static assets now.