set VisualStudioVersion

Property introduced in Visual Studio 2012 is defaulted to the highest installed
version when called in this way and allowing imports, which causes incorrect
values under Visual Studio 2010

git-svn-id: https://svn.apache.org/repos/asf/incubator/npanday/trunk@1453369 13f79535-47bb-0310-9956-ffa450edef68