add enable metadata-json-lint gem

Gemfile

@@ -6,6 +6,7 @@ group :rake do
   gem 'puppet-lint',            '~> 2.0', :require => false
   gem 'puppet-blacksmith',                :require => false
   gem 'rake',                             :require => false
+  gem 'metadata-json-lint',               :require => false
 end
 
 group :system_tests do