Choose the right dependency guise

Resolves #76

So @flagbug raised a bug where he had two versions of Caliburn.Micro in his `packages` folder:

 - 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.

This fixes that issue by implementing a proper check of the versions, and includes unit tests and an integration test showing that above scenario.