Update Unicorn template with new config for Heroku

* I recently ran into an issue after deploying to Heroku where
  `Rails.application.config.database_configuration[Rails.env]` was
  evaluating to `nil` in the staging environment. Updating
  config/unicorn.rb to the latest configuration recommended by Heroku
  fixed the problem.
* Reference:
  https://devcenter.heroku.com/articles/concurrency-and-database-connections#multi-process-servers