No NuGet for Windows Phone 8.1 Runtime App

Hi,

After creating a Xamarin.Forms Shared solution, it created projects for the 3 platforms. Windows Phone was 8.0, I wanted 8.1. I added another project(Windows Phone Runtime App 8.1), but I couldn't find a reference package for Xamarin.Forms. However I did, if I chose a Windows Phone Silverlight 8.1 app, but this isn't an option for my app.
Any ideas why this is?

Thanks in advance for any help.