Upgrade to Capistrano 3

Same migration process as https://github.com/square/kochiku-worker/pull/14

> Existing users will need to migrate their custom cap recipes to Capistrano 3 syntax. Instead of placing installation specific config in `config/deploy.custom.rb`, custom cap tasks should be added to `.cap` files in  `lib/capistrano/tasks/` and cap config should go in `config/deploy/production.rb`.

@Shenil