The problem is...

unable to establish master SSH connection: the authenticity of the tar
+get host can't be established, the remote host public key is probably
+ not present on the '~/.ssh/known_hosts' file
[download]

The thing is that Net::SSH2 is insecure by default. It doesn't check the public key of the remote host unless the programmer explicitly does it and AFAIK, nobody actually does it.

On the other hand Net::OpenSSH always checks the remote host key. I could change Net::OpenSSH::Compat::SSH2 to behave in the same way, but well, that wouldn't let me sleep at night... promoting such sloppy programming practice!!!

Anyway, as a work around just log into the remote machine from the command line using ssh so that the remote host key is stored in ~/.ssh/known_hosts and from that moment your code should work.