Daemonize puma on production

We're already doing this thanks to capistrano3-puma, but it's easy to
forget this option when starting puma manually.

config/puma.rb

@@ -13,6 +13,7 @@ state_path "#{rails_root}/tmp/pids/puma.state"
 stdout_redirect "#{rails_root}/log/puma_access.log", "#{rails_root}/log/puma_error.log", true
 
 bind "unix://#{rails_root}/tmp/sockets/puma.sock"
+daemonize
 
 threads 0, 16
 workers 2