Capistrano 3

@telent @simplybusiness/developers 

Enable this capistrano ext to be used under capistrano 3
I did indeed open source a fork of the gem, because it allowed me to fork out some new features quickly when I was trying some stuff. I can take it down if needed?

Cap 3 support is something I've been meaning to get working in Capistrano-spec, I've got some stuff commited somewhere, let me have a look...

BTW, have you guys thought of using https://github.com/pitluga/supply_drop? Already has Cap 3 support :smile: