Dropped ReactiveUI references from Shimmer.Client and Shimmer.Core

The [Shimmer.Client](http://www.nuget.org/packages/Shimmer.Client) and [Shimmer.Core](http://www.nuget.org/packages/Shimmer.Core) packages still have a reference to ReactiveUI although it isn't needed. I removed the reference so the NuGet package can be used with RxUI 5