Carthage ssh_jump_host does not appear to support strings

[srak289]

It appeared that ssh_jump_host should be able to be a string but ssh_online attempts to call ssh_jump_host.ssh_online which is not an attribute of str

[hartmans]

I agree this is a bug. I think the correct fix is to only call ssh_online on the jump_host if it hasattr('ssh_online')