Use Puma application server

- https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server

config/puma.rb

@@ -0,0 +1,23 @@
+#Please read the following before making any changes
+#1. https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server
+#2. https://devcenter.heroku.com/articles/concurrency-and-database-connections#threaded-servers
+
+workers Integer(ENV['WEB_CONCURRENCY'] || 2)
+threads_count = Integer(ENV['MAX_THREADS'] || 1)
+threads threads_count, threads_count
+
+preload_app!
+
+rackup      DefaultRackup
+port        ENV['PORT']     || 3000
+environment ENV['RACK_ENV'] || 'development'
+
+on_worker_boot do
+  # Valid on Rails up to 4.1 the initializer method of setting `pool` size
+  ActiveSupport.on_load(:active_record) do
+    config = ActiveRecord::Base.configurations[Rails.env] ||
+                Rails.application.config.database_configuration[Rails.env]
+    config['pool'] = ENV['DB_POOL'] || 5
+    ActiveRecord::Base.establish_connection(config)
+  end
+end