use puma

Procfile

@@ -1 +1 @@
-web: bundle exec rails server -b 0.0.0.0 -p ${PORT:-3000}
+web: bundle exec puma -C config/puma.rb

config/puma.rb

@@ -0,0 +1,16 @@
+# https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server
+workers Integer(ENV['WEB_CONCURRENCY'] || 2)
+threads_count = Integer(ENV['RAILS_MAX_THREADS'] || 5)
+threads threads_count, threads_count
+
+preload_app!
+
+rackup      DefaultRackup if defined?(DefaultRackup)
+port        ENV['PORT']     || 3000
+environment ENV['RACK_ENV'] || 'development'
+
+on_worker_boot do
+  # Worker specific setup for Rails 4.1+
+  # See: https://devcenter.heroku.com/articles/deploying-rails-applications-with-the-puma-web-server#on-worker-boot
+  ActiveRecord::Base.establish_connection
+end