There are three reasons why I would suggest that you don't emphasise the proficiency in Ruby. The first is, as mentioned above, that you're probably overestimating the effort required to switch platform at this point. The second is that a good programmer who just happens to have no/limited experience with Ruby will probably be able to adapt pretty fast, provided he has experience with similar technologies. By limiting your search to those that already know Ruby well, you're avoiding a lot of potential.
But the main reason why I think it's a bad idea, is because you are making decisions that I believe is the domain of a CTO to make. By doing so, you are sending a signal that you don't respect/trust him to make that decision. That is going to make the type of person you'd want onboard think twice about joining. I'm not suggesting that you actually feel this way (I don't know you, so I couldn't pass that judgement), but you should be aware of the way it will read to - at least some - people in your target demography.