Reenable AutobahnTestAppAspNet4

[JunTaoLuo]

As part of migrations to VS 2017, I temporarily disabled building the app which targets 4.6.1. Once we can build this sample again, we need to add it back to the solution and start building it again. https://github.com/aspnet/WebSockets/blob/0e1dd78771f113db5451505bcfce02f27a97d48c/samples/AutobahnTestAppAspNet4/AutobahnTestAppAspNet4.csproj.aspnet4

[aspnet-hello]

This issue was moved to dotnet/aspnetcore#2476