Allow restarting puma with rails restart

So now the way to restart the application does not depend on whether we
use puma or passenger.

What this plugin actually does is restarting puma when we touch the
`tmp/restart.txt` file, which is what `rails restart` does.

config/puma.rb

@@ -20,6 +20,7 @@ workers 2
 preload_app!
 
 restart_command "bundle exec --keep-file-descriptors puma"
+plugin :tmp_restart
 
 on_restart do
   puts "Refreshing Gemfile"