Update prelude-packages.el

New URL, according to http://melpa.org/#/getting-started

core/prelude-packages.el

@@ -36,7 +36,7 @@
 (require 'package)
 
 (add-to-list 'package-archives
-             '("melpa" . "http://melpa.milkbox.net/packages/") t)
+             '("melpa" . "http://melpa.org/packages/") t)
 ;; set package-user-dir to be relative to Prelude install path
 (setq package-user-dir (expand-file-name "elpa" prelude-dir))
 (package-initialize)