Handle `.xccurrentversion` file when adding versioned data models to project

These changes implement and spec the handling of `.xccurrentversion` files when adding versioned data models to projects.

This was done to try and fix this issue in https://github.com/CocoaPods/CocoaPods/issues/1288
However something is still stopping the xcdatamodeld contents from being processed.

Issue: https://github.com/CocoaPods/Xcodeproj/issues/81