[NPANDAY-578] fix determination of framework versions

This will attempt to discover them in both v4.0 and v2.0 Microsoft.Build
libraries, so we need to make sure it doesn't throw an exception looking for
newer versions in old libraries and instead just skips them.

git-svn-id: https://svn.apache.org/repos/asf/incubator/npanday/trunk@1453698 13f79535-47bb-0310-9956-ffa450edef68