In case this happens to you, thought I would provide some help.
After updating Xcode to version 8 today while continuing to run on Mac OS X El Capitan (10.11), I went to my Mac OS RubyMotion apps, ran 'rake clean' and then BOOM!
ERROR! To use specified OSX SDK version, it requires running on host of same OSX version or higher. But you are running OSX 10.11.6
I have hit this a couple of times when upgrading Xcode. Apple now releases Xcode with only the latest Mac SDK and no way to get the older SDKs. MacOS sierra (10.12) doesn't come out for a week so you are in something of a catch-22 if you don't want to also upgrade to the beta.
First you need the 'old' SDKs, go here: https:/github.com/phracker/MacOSX-SDKs you can either clone or just download individuals SDK versions. I downloaded the 10.11 SDK (I already had 10.9 and 10.10)
Next you need to copy the SDK into the Xcode Frameworks here: /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs
Finally to run in this mixed environment, you will need to ensure your app remains targeted to El Capitan by adding these two lines to your Rakefile:
app.sdk_version = '10.11'
app.deployment_target = '10.11'
in the Motion::Project::App.setup block
Hope this saves someone some time. Note once you do upgrade to Sierra, you can simply remove the sdk_version and deployment_target from the Rakefile and your app will once again compile against the latest OS release.