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.
Seems like a helper method that can read ENV or get data from `fetch` would be kind of cool since it would make sure these were all called identically.