Broaden sshkit dependency to >= 1.2.0

SSHKit 1.3.0 was recently released with a huge speed improvement due by reusing ssh connections.

`capistrano-rbenv` was preventing the upgrade since it was locked to a 1.2.x version of sshkit. I would assume it would be safe to use any 1.x version, so this broadens the requirement a little.

**Update**: I changed it to `>= 1.2.0` instead of `~1.3`