Remove dodgy use of $( array ).last()

Per the docs, these are only meant to be used on arrays of DOM elements.
jQuery might one day assign a different meaning to arrays of strings,
and then we could have some security issues or weird breakage.

(imported from commit 545eee1e9c6955556d5c4bda30cd6db0dce19c60)

zephyr/static/js/ui.js

@@ -434,7 +434,8 @@ $(function () {
         items: 4,
         matcher: function (item) {
             // Assumes email addresses don't have commas or semicolons in them
-            var current_recipient = $(this.query.split(/[,;] */)).last()[0];
+            var recipients = this.query.split(/[,;] */);
+            var current_recipient = recipients[recipients.length-1];
             // Case-insensitive (from Bootstrap's default matcher).
             return (item.toLowerCase().indexOf(current_recipient.toLowerCase()) !== -1);
         },