Seems like there's an issue with using Ruby 2.3.0 with CocoaPods. Looks like the fix is in master, but I'm curious how long it takes for fixes like this to trickle down to RubyMotion. I thought I remembered hearing that there were issues with newer versions of CocoaPods so I don't think I'm even using the latest version. Maybe I'm wrong.
Use gem install motion-cocoapods --pre
gem install motion-cocoapods --pre