Some tests of ours require to be running against an SSH server.
Currently, we simply run against the SSH server provided and started by
Travis itself. As our Linux tests run in a sudo-less environment, we
have no control over its configuration and startup/shutdown procedure.
While this has been no problem until now, it will become a problem as
soon as we migrate over to newer Precise images, as the SSH server does
not have any host keys set up. Luckily, we can simply set up our own
unpriviledged SSH server. This has the benefit of us being able to
modify its configuration even in a sudo-less environment.

This commit sets up the unpriviledged SSH server on port 2222.