Fixed a reference in test project

GalaSoft.MvvmLight/Tests/PCL/GalaSoft.MvvmLight.Test (PWPSL81)/GalaSoft.MvvmLight.Test (PWPSL81).csproj

@@ -388,7 +388,7 @@
       <HintPath>..\..\..\packages\CommonServiceLocator.1.3\lib\portable-net4+sl5+netcore45+wpa81+wp8\Microsoft.Practices.ServiceLocation.dll</HintPath>
     </Reference>
     <Reference Include="System.Windows.Interactivity">
-      <HintPath>..\..\..\_Binaries\Release\WPSL81\System.Windows.Interactivity.dll</HintPath>
+      <HintPath>..\..\..\External\WPSL81\System.Windows.Interactivity.dll</HintPath>
     </Reference>
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath)\Microsoft\$(TargetFrameworkIdentifier)\$(TargetFrameworkVersion)\Microsoft.$(TargetFrameworkIdentifier).$(TargetFrameworkVersion).Overrides.targets" />