add metadata-json-lint gem

Needed by rake metadata.json validation target.

Gemfile

@@ -13,20 +13,21 @@ else
 end
 
 group :development, :test do
-  gem 'rake',                    :require => false
+  gem 'rake',                     :require => false
   # https://github.com/rspec/rspec-core/issues/1864
   gem 'rspec', '< 3.2.0', {"platforms"=>["ruby_18"]}
-  gem 'puppetlabs_spec_helper',  :require => false
-  gem 'puppet-lint', '>= 1.1.0', :require => false
-  gem 'puppet-syntax',           :require => false
+  gem 'puppetlabs_spec_helper',   :require => false
+  gem 'puppet-lint', '>= 1.1.0',  :require => false
+  gem 'puppet-syntax',            :require => false
   gem 'rspec-puppet', '~> 2.1.0', :require => false
+  gem 'metadata-json-lint',       :require => false
 end
 
 group :beaker do
-  gem 'serverspec',              :require => false
-  gem 'beaker',                  :require => false
-  gem 'beaker-rspec',            :require => false
-  gem 'pry',                     :require => false
+  gem 'serverspec',               :require => false
+  gem 'beaker',                   :require => false
+  gem 'beaker-rspec',             :require => false
+  gem 'pry',                      :require => false
 end
 
 # vim:ft=ruby