This updates target framework to netcorapp2.0 and updates dependencies (Machine.Specifications and Microsoft.NET.Test.Sdk). This, in combination with #42, makes the tests possible to run for me on Windows as well as Linux.

I should say I find the combinations of .NET Core and .NET Standard very confusing, so I apologize in advance if I've messed something up. Also, some tool appears to have changed indentation on everything 😞 .