Rewritten for Capistrano 3

@relistan I've hacked about with this and ended up with a working Capistrano 3 version that behaves pretty much identically to the original. Unfortunately capistrano-spec still seems to be cap 2 only. I can't find a single cap 3 plugin that has tests, so reluctantly I've disabled the specs until I can either think of a way to test it, or a testing framework emerges.
It seems to be the pattern for cap 3 plugins - see e.g. https://github.com/capistrano/rails/tree/master/lib, the capistrano-rails.rb is empty, and you are expected to explicitly `require capistrano/rails` from Capfile.