Contextual links fix for Boxes in Display Suite block fields

Using Display Suite, I sometimes create a Display Suite block field containing a Box block. This breaks the Box contextual links if there are multiple such fields on one page. Turns out the jQuery selector tries to find the block parent using .parents(".block"), however in the above scenario, both DS and Boxes outputs a container with a class of "block".

The attached patch simply changes the jQuery selector to .closest, which finds the correct block container.