WIP: Choose the right dependency guide

So @flagbug raised a bug where he had two versions of Caliburn.Micro 

 - 1.4.1
 - 1.5.1

And in his application the nuspec would specific 1.5.1

But when he creates the release package it chooses 1.4.1.

So let's fix it. [You probably want to look here for the actual lookup code](https://github.com/github/Shimmer/blob/choose-latest-version-nuspec/src/Shimmer.Core/ReleasePackage.cs#L216)