Make solution file compatible with VS 2010

Without this change, VS 2010 Pro rejects the solution file for having being created by a newer version of VS. I don't know whether this would have side effects of any kind, but I saw a former VS PM do this on GitHub to solve the same problem for a different project, so I guess it's more or less ok.

Related to https://github.com/fsharp/fsharpbinding/issues/336