Update to latest tooling #65
Description
Bring this repo up-to-date w.r.t. the MSBuild version, the StyleCop version, and the Castle.Core / Moq / xUnit versions used in tests. Do not change product code, dependencies, or runtime targets.
This includes a small quirk on the test side: Latest xUnit packages depend on various .NET Standard packages if the test project targets anything earlier than .NET 4.5.2. Update the test projects to target .NET 4.5.2 to avoid those dependencies. We're only testing on more-recent Windows, and therefore desktop .NET, versions anyhow.
See PR aspnet/WebHooks#167 for a smaller variant of this work.