Boxen: Allow provisioning with SSH-based repository URLs

[radeksimko]

If you use git@github.com:org_name/our-boxen as a default URL for upstream remote, vagrant will show following error:

==> boxen: --> Preparing to auto-update...
==> boxen: Host key verification failed.
==> boxen: fatal: Could not read from remote repository.
==> boxen:
==> boxen: Please make sure you have the correct access rights
==> boxen: and the repository exists.
==> boxen: --> Skipping auto-update, because running `git fetch` failed
==> boxen:
==> boxen: Well that's disappointing! Are you having network trouble? Is github.com
==> boxen: under attack? Given the potential for all sorts of boxen-y things failing
==> boxen: without the ability to pull from github, I'm going to abort for now.
==> boxen:
==> boxen: Check that you can hit github, see if there are any ongoing issues with
==> boxen: the site and/or your network, and try again ASAP!
==> boxen:
==> boxen: For more info on how the simple git-based updates work, have a gander at
==> boxen: https://github.com/boxen/our-boxen/blob/master/docs/updates.md
==> boxen:
==> boxen: Would you like to run Boxen anyway, without the latest updates? [y/N]

Because it doesn't have any key to use when reaching github.com over SSH.

Maybe Vagrant has some kind of key host->guest forwarding feature or maybe we can just simply skip this check the same way as intended in #1 ?