Fixes javascript error with SelectMultiple-widget.

Error: Option 'multiple' is not allowed for Select2 when attached to a <select> element.
http://127.0.0.1:8000/static/select2/js/select2.js?v=1
Line 1289

Even though the Select2 docs[1] say

   When Select2 is attached to a select element this value will be ignored and
   select's multiple attribute will be used instead.

this is not ignored, but throws an error.

[1] http://ivaynberg.github.io/select2/#documentation