Upgrade to Capistrano 3

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`.

This is a pretty disruptive change but I don't want to be on Capistrano 2.x forever.

Once this is merged I will update the instructions on the wiki. I will also be upgrading Kochiku web to Capistrano 3.x as well.

@square/kochiku-contributors