Cannot build in Visual Studio 2017

[lookatmike]

When you try to load any of the solutions in the /csharp/build folder with anything other than VS2015 (or, perhaps, without at least having VS2015 installed), you'll see a bunch of errors like this for each project:

The imported project "C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\15.0\Microsoft.VisualStudioVersion.v14.Common.props" was not found.

This would seem to come from the following import line that most of the projects have:

<Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.VisualStudioVersion.v14.Common.props" />

That appears to be a VS2015-specific thing. Would it be possible to replace that with Microsoft.Common.props instead? Should be backwards-compatible, and would make it easier to work on the project outside of VS2015. I'll submit a PR for that, but I'm open to alternatives.