Ensure deploying user has added their ssh-key

Recently had an issue with someone trying to deploy without an ssh-key added. It's much more ideal to detect this issue, rather than have a comment within the ruby source here telling them what to do. The lack of working ssh-key forwarding can be tricky to debug in certain cases such as if other keys are present on the target machine (particularly if those are GitHub deploy keys for a *different* repo).

This makes the deployment system easier to setup for first time users.