fix the bug initSelection not being called when multiple=true

the problem is ui-select2 init select2 plugin before it init the element val, and select2 do not call initSelection if the val of the element is empty.
so i change the line order to first init the element val and then start select2

plunker that show the problem
http://plnkr.co/edit/A9ZhuLdDiGBqLpNWk2eY?p=preview