If you aren't terribly unhappy there, tell them what has happened and ask to come back. There is no shame in that at all - in fact it puts you in a pretty good position at your old job (because they now know your skills are valued elsewhere).

Another possibility is telling them what has happened, but say you still planning on moving so offering to stay for another month.

OTOH, there is plenty of RoR work around, so I'd be surprised if you can't pick something else up pretty easily.