Make sure all samples are compiling against latest RxUI

[glennawatson]

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You can in PackageReference's use the * notation in the Version.

Eg

<ProjectReference Include="ReactiveUI" Version="*" />

If you use this notation for any RxUI based dependencies it'll make sure we are on latest.

That way we can trigger sample compiles after any new release of ReactiveUI to make sure the samples still at least compile and pass any unit tests they have.