Default short_version to be equal to version

Reduces friction for the user:

* Most users are not interested in setting a separate `CFBundleShortVersionString`
* The string is required to be unique for every version released to the App Store
* The setting is not generated during `motion create`, and it is not used in any of the examples in [RubyMotionSamples](https://github.com/HipByte/RubyMotionSamples)