To @dchersey and anyone else reading this...don't equate "maintenance mode" with being inactive (or stagnant). Naturally, when building something new, there is a lot of activity at first, then as it matures, there is less of a need to make major changes.
The RubyMotion community is still active and thriving. Personally, I have GitHub repo access to a number of these popular gems, so if you find a bug and submit a PR, I can probably merge it and release a new version.
MotionKit and CDQ are both great gems for layout and client-side data storage for iOS apps. The RM community has produced some great gems for iOS development. Realistically, Android development has only been possible recently. So eventually the community will want to produce gems that are cross-platform. Until then, I would say that the iOS specific gems are still a great option.
I would caution you against switching to Flow too soon. It definitely has a lot of potential, but I think there are still a lot of missing pieces. For example, it does not include a data storage equivalent to CDQ, so don't use that as a reason to switch. I think it would make sense to create something like this as a separate cross-platform gem since not every app is going to need this type of data storage.
In general, if you create your app using good abstractions, you will be able to reuse a lot of your code regardless of which framework/gems you are using.