Fix issue introduced by 20950e0f0cbbaa2ed9ddb3306320a69d4691c87d.

Selected items and other invisible items were not ignored by highlightUnderEvent when calculating index.
This caused highlighting of incorrect items when the list contained hidden items.

select2.js

@@ -1126,7 +1126,7 @@ the specific language governing permissions and limitations under the Apache Lic
         highlightUnderEvent: function (event) {
             var el = $(event.target).closest(".select2-result-selectable");
             if (el.length > 0 && !el.is(".select2-highlighted")) {
-        		var choices = this.results.find('.select2-result');
+        		var choices = this.results.find('.select2-result:visible');
                 this.highlight(choices.index(el));
             } else if (el.length == 0) {
                 // if we are over an unselectable item remove al highlights