Set RAILS_ENV to production and execute commands via exec

RAILS_ENV: When the procfile is being exported to the god config format, we can assume that the processes should run in the production environment. If that's a bad idea, we could replace it with `RAILS_ENV => ENV['RAILS_ENV']` and let somebody else decide.

exec: God is sending signals to the processes to terminate them. If the first subprocess is just a shell script, which it is in this case, the real process does not receive the signal properly. Therefore the shell scripts must replace their process via exec.