(Maint) Add windows gem dependencies

Hiera has dependencies on windows-specific gems that weren't expressed
in the Gemfile. This updates the gemfile, and pins the versions to what
we are currently shipping.
(Maint) Don't require json gem on non-ruby platforms

The json gem contains native extensions which won't compile on windows spec
boxes without the devkit installed.

This commit makes the json gem conditional upon the `ruby` platform, as is
done in facter and puppet.
(Maint) Skip test on Windows

A spec test fails on Windows, because ruby on windows doesn't exhibit the
crazy behavior that the test is trying to assert. Specifically, trying to
open /dev/null on Windows returns ENOENT, not TypeError.

This commit skips the test on windows, and removes the expect { }.to_not
business.
Merge branch 'maint/stable/gemfile-dependencies' into stable

* maint/stable/gemfile-dependencies:
  (Maint) Skip test on Windows
  (Maint) Don't require json gem on non-ruby platforms
  (Maint) Add windows gem dependencies

Closes GH-160