(FACT-570) Remove windows-pr dependency

Previously, we used the windows-pr gem to detect which windows version we
were running on, and then either called into the win32-security gem for
windows versions < 6.0 (2003), or used windows-pr gem to determine if
we were running with elevated privileges.

However, the `Win32::Security.elevated_security?` method now correctly
handles Vista and up, so we can just call that directly, avoiding
windows-pr, and in the process eliminating win32-api.
(maint) Ensure gems are not required in Gemfile

Previously, I omitted the `require => false` line in the Gemfile,
which means bundler will magically require the library. That's ok
when running from source, but if Facter is not running from source
and forgets to explicity require a gem, then Facter will fail hard.

This commit ensure Facter explicitly requires all Windows dependencies.
Merge branch 'ticket/facter-2/FACT-570-remove-windows-pr' into facter-2

* ticket/facter-2/FACT-570-remove-windows-pr:
  (maint) Ensure gems are not required in Gemfile
  (FACT-570) Remove windows-pr dependency