Xamarin Runner cannot reference NUnit NuGet Package

[rprouse]

As initially reported by @ChrisMaddock , since the .NET Standard target was added to the NuGet package, the Xamarin projects cannot reference the package. I have confirmed this and tracked it down to the reference to Microsoft.NETCore.Portable.Compatibility in the package. The package lists this as a dependency because the framework references it, but it only references it because of some unused using statements.