Maint: Don't try to install json gem on Windows

Previously, `bundle install` would fail on windows without the ruby devkit
installed. As this is not something we ship with, confine the gem to only be
installed on ruby platforms, i.e. not windows.

Also running `bundle install --without development test` followed by `bundle
exec facter` on all platforms would fail due to `facter` being in the
development/test group.