cap 3 support

Here's what I get when I try to use cap-ssh with cap 3:

```
[1] pry(main)> require 'cap-ssh'
NoMethodError: undefined method `instance' for Capistrano::Configuration:Class
from /Users/milesmatthias/.rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/cap-ssh-0.1.1/lib/cap-ssh.rb:3:in `<top (required)>'
```
