refact(select): use prop to modify the select property

jQuery suggests using `prop` rather than `attr` to modify the `select` property of an element.
You can see the full list of migration warnings for jQuery:
https://github.com/jquery/jquery-migrate/blob/master/warnings.md

Closes angular#4107
Closes angular#4122

Author: colincasey

src/ng/directive/select.js

@@ -553,7 +553,7 @@ var selectDirective = ['$compile', '$parse', function($compile,   $parse) {
                   // rather then the element.
                   (element = optionTemplate.clone())
                       .val(option.id)
-                      .attr('selected', option.selected)
+                      .prop('selected', option.selected)
                       .text(option.label);
                 }