Don't reference MS-Rx in Mono projects.

This should fix-up the NuGet's until Rx has Xamarin support. The Mono projects will need to use the Xamarin component store to pull in Xamarin's Rx.

This does nothing to solve the PCL incompat, but allows NuGet to add RxUI to a Xamarin.iOS/Android project w/o pulling in the wrong Rx references.