(Maint) Pin windows gems to match what we ship

The win32-dir gem has a dependency on ffi, which was recently updated to
1.9.3 but does not provide a precompiled version. As a result, the gem will
attempt to compile native extensions, which will fail on our spec boxes
(since the dev kit is purposefully not installed).

This commit pins ffi to 1.9.0, updates win32-dir and windows-pr to the
version that we ship with, and pins each gem to an exact version matching:

https://github.com/puppetlabs/puppet-win32-ruby/tree/1.9.3/ruby/lib/ruby/gems