(FACT-570) Remove windows-pr dependency

Previously, we used the windows-pr gem to detect the current windows
version, and either called into the win32-security gem for 2003
(version 5.2), or Vista and up (version 6.0 and up). The logic is
different due to UAC introduced in Vista.

In 2003, a user is "elevated" if the process token contains the local
Administrators SID, e.g. they're a member of the local Administrators
group.

In Vista and up, every process has two tokens, and only if the process
is running with the privileged token, e.g. Run as Administrator, is it
truly elevated.

Earlier versions of the win32-security gem, before it was ffi'ed, only
worked correctly on 2003.

However, newer versions in the 0.2.x series do support Vista and up, so
this commit removes our version of the code, and calls win32-security
directly. As a result, we can remove our dependency on windows-pr,
and the win32-api gem it depends on.