Update System.Windows.Interactivity.dll

A couple other frameworks have updated System.Windows.Interactivity.dll for .NET 4.5 to the version 3.0.40218.0. I've noticed that the version used by my project is dependent on the last NuGet package to have been updated (if it's MahApps.Metro, it uses the old version; if it's a different framework, it uses the new version). The first change included updates the DLL to the latest version.

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Fixes #505