Improvements to determining the current version of a versioned CoreData model

[rpassis]

The current implementation for generating versioned CoreData models and selecting the current active version uses alphabetical sorting. This can work for most simple projects but is likely to break for less conventional setups.

A better solution is to parse the .xccurrentversion file which is included inside the CoreData versioned model. This file is in plist format and is created automatically by Xcode.

This is a fix for issue #362

[rpassis]

One thing to note is that the alphabetical order implementation here will break even for very basic use cases. A very simple versioned model could look like this

Model.xcdatamodel
Model 1.xcdatamodel
Model 2.xcdatamodel

When applying the existing sort logic and taking the last item as the current CoreData model you will end up with

Model 1.xcdatamodel
Model 2.xcdatamodel
Model.xcdatamodel

And Model.xcdatamodel incorrectly set as the current version.

I'd suggest we should just remove the alphabetical fallback completely but I can't be certain that all CoreData models do include a .xccurrentversion file indicating what the current version is (based on my own tests it does but this doesn't seem to be documented anywhere).

[yonaskolb]

Thanks @rpassis, looks great!

[yonaskolb]

Can we keep the if as opposed to the guard? Not being a xcdatamodeld is not a failing case, and we may handle other filetypes differently in the future as well.
Additionally it will make these diffs easier to read.

[rpassis]

@yonaskolb thanks for the feedback! I've addressed your comment and pushed a new commit.

[yonaskolb]

Thanks @rpassis!