heavily customise our unicorn upstart config

* stop using the upstart meta dep, it's too restrictive
* the old configs were configured as a task - they could start the
  unicorns on boot, but they couldn't monitor the process and restart
  it if it stopped for any reason
* This is a ercognised issue - upstart can't track processes that fork
  more than twice and unicorn forks every time we send it USR2 on deploy
* This new config is inspired by [1] and tricks upstart into monitoring
  a looping shell script. The shell script monitors the unicorn PID
  files and exists if the process has stopped
* This should mean:
  * unicorns start on boot
  * can be reloaded with USR2 on deploy
  * will be restarted by upstart if they ever die
  * can be started and stopped with "initctl start" and "initctl stop"

[1] http://rubyforge.org/pipermail/mongrel-unicorn/2013-August/001829.html