Format CFBundleShortVersionString to spec

[DMcNamara]

According to Apple:

The release version number is a string composed of three period-separated integers.

This change makes an attempt to pull out a string that is valid for CFBundleShortVersionString from the input, returning the input if no matches are found.

This is a potential fix for #41

[stovmascript]

Hey @DMcNamara, thanks for the PR! Based on the tests, it looks like this change will make it return the previous version string when making/bumping a pre-release version. Will this cause problems when trying to release to TestFlight/App Store, or can that be handled by bumping the build number?

[DMcNamara]

In our experience, putting anything in the CFBundleShortVersionString that is not int.int.int causes issues. We version things like 1.2.3-alpha.0, 1.2.3-alpha.1, etc. For all of those pre-release builds we upload to TestFlight with a CFBundleShortVersionString of 1.2.3 and increment the CFBundleVersion. Submitting it to the App Store also works as expected with just bumping the CFBundleVersion.

I'm not an Apple App Development expert by any means, so maybe I'm missing something but we've been using this process for a few months and it's been working.