add cleanup task.... capistrano should have this by default, yet here we...

@Shopify/data-engineers 

capistrano is supposed to have a deploy:cleanup task, but it doesnt actually seem to have one. I merged one over from the capistrano repository and scheduled it to run after deploy restart.

based on a quick knife, it looks like we were never running this as there are over 60 old releases on some boxes eating over 11G of disk space.