Get correct OSVersion when running on Windows 8.1

fixes #114
Merge branch 'patch-2' of https://github.com/jesperll/raygun4net into jesperll-patch-2
support medium trust for OS version

retain the original version string location code but if we are in full
trust then attempt to obtain the correct version string using WMI
Merge pull request #116 from jesperll:patch-2