[X-Pack] Add license namespace when using xpack

Author: picandocodigo

elasticsearch-xpack/lib/elasticsearch/xpack.rb

@@ -75,6 +75,10 @@ def data_frame
       def ilm
         @ilm ||= xpack.ilm
       end
+
+      def license
+        @license ||= xpack.license
+      end
     end
   end
 end if defined?(Elasticsearch::Transport::Client)