Use SSH url for cloning to prevent login prompts.

[kleinmann]

I noticed when backing up private repositories that I get asked for username and password. As github-backup is meant for cronjobs, this is not what I wanted :-) I changed it to use the SSH url because then ssh-agent provides the authentication.

I'm not sure if this is a change for the majority of users, just close it if it creates problems for other cases :-)

[ericlathrop]

I've got a branch I haven't pushed yet that has command-line flags. This change would be fine behind a flag. Something like --private.