make getPlaceholder() compatible with jquery 1.4

(doesn't check data- attrs when using .data())

select2.js

@@ -809,7 +809,10 @@
         },
 
         getPlaceholder: function () {
-            return this.opts.element.attr("placeholder") || this.opts.element.data("placeholder") || this.opts.placeholder;
+            return this.opts.element.attr("placeholder") ||
+                this.opts.element.attr("data-placeholder") || // jquery 1.4 compat
+                this.opts.element.data("placeholder") ||
+                this.opts.placeholder;
         },
 
         /**