Reference fixed Roslyn version

[ltrzesniewski]

This follows my remark from here:

Oh, I didn't notice this earlier, but I think the Microsoft.CodeAnalysis.CSharp package should be rolled back to v4.0.1 and frozen at that version unless there's a good reason to use a newer one, since referencing a newer version requires the user to have a .NET SDK or Visual Studio version which supports it. See here for the version map.

The problem is that it won't play along with tools which always suggest to bump the packages to the latest versions. 😞

So this ensures the analyzer supports older compilers, but runs tests with the latest one. Contrary to my previous remark, IDEs won't suggest updating Microsoft.CodeAnalysis.CSharp.

What do you think @tom-englert?