Don't rely on :selected, makes Select2 compatible with jQuery builds without Sizzle.

select2.js

@@ -2119,7 +2119,7 @@ the specific language governing permissions and limitations under the Apache Lic
         isPlaceholderOptionSelected: function() {
             var placeholderOption;
             if (!this.getPlaceholder()) return false; // no placeholder specified so no option should be considered
-            return ((placeholderOption = this.getPlaceholderOption()) !== undefined && placeholderOption.is(':selected'))
+            return ((placeholderOption = this.getPlaceholderOption()) !== undefined && placeholderOption.prop("selected"))
                 || (this.opts.element.val() === "")
                 || (this.opts.element.val() === undefined)
                 || (this.opts.element.val() === null);
@@ -2133,7 +2133,7 @@ the specific language governing permissions and limitations under the Apache Lic
             if (opts.element.get(0).tagName.toLowerCase() === "select") {
                 // install the selection initializer
                 opts.initSelection = function (element, callback) {
-                    var selected = element.find(":selected");
+                    var selected = element.find("option").filter(function() { return this.selected });
                     // a single select box always has a value, no need to null check 'selected'
                     callback(self.optionToData(selected));
                 };
@@ -2303,7 +2303,7 @@ the specific language governing permissions and limitations under the Apache Lic
             if (this.select) {
                 this.select
                     .val(val)
-                    .find(":selected").each2(function (i, elm) {
+                    .find("option").filter(function() { return this.selected }).each2(function (i, elm) {
                         data = self.optionToData(elm);
                         return false;
                     });
@@ -2398,7 +2398,7 @@ the specific language governing permissions and limitations under the Apache Lic
 
                     var data = [];
 
-                    element.find(":selected").each2(function (i, elm) {
+                    element.find("option").filter(function() { return this.selected }).each2(function (i, elm) {
                         data.push(self.optionToData(elm));
                     });
                     callback(data);