Fix symlink support for older Puppet versions

 - In commit dc4e8aebb61a32292bc2bf27b31a422902878294, a change was
   introduced to enable the 'spec_prep' task to work properly on
   Windows.  Unfortunately this was done without appropriate
   consideration for the versions of Puppet that spec_helper need to
   support, and Puppet::FileSystem symlink support was added in Puppet
   3.5, via https://github.com/puppetlabs/puppet/commit/938b4e7d495f5f15557110040458171e832cecf2

   While it's true that the symlink code was initially added to Puppet
   in 3.4, we'll not take that into consideration and just rely
   on the existence of Puppet::FileSystem as our heuristic for
   Windows symlink support (given the code was structured differently
   in prior versions)

   Additionally emit an upgrade failure on Windows when symlinks are not
   supported.

   The is_windows check is equivalent to how Puppet and Ruby itself
   test for Windows.