Thanks @tilmanx, that worked around the issue for me. After a botched Xcode 7.1 install, I had to do delete Xcode and install from scratch. Then I could not find the actual IOS 9.0 SDK for download, so had to "simulate" it with the symlink above.
I also had to work around a similar issue for the Simulator. Even though I had downloaded the 9.0 simulator from Xcode > Preferences > Downloads, RubyMotion Starter 4.4 was not recognizing it. I also did:
sudo ln -s iPhoneSimulator.sdk iPhoneSimulator9.0.sdk
And now I can build and simulate.
This seems a little fragile, but should hold until HipByte releases an update in sync with current SDKs.
[Edit - this worked for me to run in simulator, but subsequently failed on rebuilding cocoa pods. This was a temporary workaround, not a solution. The new release of RubyMotion Starter 4.5 solves the problem.]