Update select2.sortable.js

Added support for group of elements in select2SortableOrder function.

Check if the select is already select2-ed in select2Sortable to prevent previous select2 configuration loss.

Modified select2SortableDestroy to destroy only jQuery UI sortable and non not select2 entirely.
Merge https://github.com/matteopoile/select2-sortable into matteopoile-master