Add jQuery UI Droppable as module requirement

Author: nikcub

src/gridstack.js

@@ -11,7 +11,8 @@
             'jquery-ui/form', 'jquery-ui/ie', 'jquery-ui/keycode', 'jquery-ui/labels', 'jquery-ui/jquery-1-7',
             'jquery-ui/plugin', 'jquery-ui/safe-active-element', 'jquery-ui/safe-blur', 'jquery-ui/scroll-parent',
             'jquery-ui/tabbable', 'jquery-ui/unique-id', 'jquery-ui/version', 'jquery-ui/widget',
-            'jquery-ui/widgets/mouse', 'jquery-ui/widgets/draggable', 'jquery-ui/widgets/resizable'], factory);
+            'jquery-ui/widgets/mouse', 'jquery-ui/widgets/draggable', 'jquery-ui/widgets/droppable',
+            'jquery-ui/widgets/resizable'], factory);
     } else if (typeof exports !== 'undefined') {
         try { jQuery = require('jquery'); } catch (e) {}
         try { _ = require('lodash'); } catch (e) {}