Fix usage of data-mh attribute

Before this fix, groups were miscalculated when using data-mh attribute instead of the more verbose data-match-height.

Author: Stefano Zoffoli

jquery.matchHeight.js

@@ -135,7 +135,7 @@
         // generate groups by their groupId set by elements using data-match-height
         $('[data-match-height], [data-mh]').each(function() {
             var $this = $(this),
-                groupId = $this.attr('data-match-height');
+                groupId = $this.attr('data-match-height') || $this.attr('data-mh');
             if (groupId in groups) {
                 groups[groupId] = groups[groupId].add($this);
             } else {
@@ -237,4 +237,4 @@
         return parseFloat(value) || 0;
     };
 
-})(jQuery);
+})(jQuery);