Add json gem

The puppet_facts gem implicitly depends on the json gem.
On Ruby 1.8.7, json is not built in. On Puppet 2.7, it is
not explicitly pulled in by the hiera gem. So we add it
here explicitly.

Gemfile

@@ -7,6 +7,7 @@ group :development, :unit_tests do
   gem 'puppet-lint',             :require => false
   gem 'simplecov',               :require => false
   gem 'puppet_facts',            :require => false
+  gem 'json',                    :require => false
 end
 
 group :system_tests do