Fix of issue 550 Select2 and CSS Transforms: dropdown menu is off position

Fix of issue 550 Select2 and CSS Transforms: dropdown menu is off position:
https://github.com/ivaynberg/select2/issues/550

jquery offset() returns incorrect value when ancestor container has css transform applied to it, causing dropdown to appear in incorrect position.

Using { top: variable.offsetTop, left: variable.offsetLeft } instead of variable.offset() fixes this issue