Use URL

Because `:rubygems` is deprecated.

Gemfile

@@ -1,6 +1,6 @@
 # -*- mode: ruby; coding: utf-8 -*-
 #
-# Copyright (C) 2012  Kouhei Sutou <kou@clear-code.com>
+# Copyright (C) 2012-2014  Kouhei Sutou <kou@clear-code.com>
 #
 # This library is free software; you can redistribute it and/or
 # modify it under the terms of the GNU Lesser General Public
@@ -15,6 +15,6 @@
 # License along with this library; if not, write to the Free Software
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 
-source :rubygems
+source "https://rubygems.org/"
 
 gemspec