Update Rx.Net dependency to match reactiveui's dependency on Rx.

build/Rx.props

@@ -1,11 +1,11 @@
 <Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <ItemGroup>
-    <PackageReference Include="System.Reactive" Version="3.0.0" />
-    <PackageReference Include="System.Reactive.Core" Version="3.0.0" />
-    <PackageReference Include="System.Reactive.Interfaces" Version="3.0.0" />
-    <PackageReference Include="System.Reactive.Linq" Version="3.0.0" />
-    <PackageReference Include="System.Reactive.PlatformServices" Version="3.0.0" />
-    <PackageReference Condition="'$(TargetFramework)' == 'net45'" Include="System.Reactive.Windows.Threading" Version="3.0.0" />
-    <PackageReference Condition="'$(TargetFramework)' == 'net461'" Include="System.Reactive.Windows.Threading" Version="3.0.0" />
+    <PackageReference Include="System.Reactive" Version="3.1.0" />
+    <PackageReference Include="System.Reactive.Core" Version="3.1.0" />
+    <PackageReference Include="System.Reactive.Interfaces" Version="3.1.0" />
+    <PackageReference Include="System.Reactive.Linq" Version="3.1.0" />
+    <PackageReference Include="System.Reactive.PlatformServices" Version="3.1.0" />
+    <PackageReference Condition="'$(TargetFramework)' == 'net45'" Include="System.Reactive.Windows.Threading" Version="3.1.0" />
+    <PackageReference Condition="'$(TargetFramework)' == 'net461'" Include="System.Reactive.Windows.Threading" Version="3.1.0" />
   </ItemGroup>
 </Project>