Plugin not showing up after upgrade

Hey there. I wrote a mini-plugin to change the subject line on Subscribe2 email plugin. The code is very simple:

<?php /* Plugin Name: WordPress Subscribe2 Email Subject Filter
Plugin URI: https://www.stevenandrewjackson.com
Description: Changes the email subject.
Version: 1.0
Author: Nicholas Andrea
Author URI: https://www.expansivewebdesign.com
License: GPL2
Copyright 2012  Nicholas Andrea  (email : [email protected])
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License, version 2, as
    published by the Free Software Foundation.
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/

function s2_subject_changes($subject) {
    return "Steven Andrew Jackson Weekly Blog Digest";
}

add_filter('s2_email_subject', 's2_subject_changes'); ?>

However, after upgrading to 3.4.1 the plugin is not showing up in the dashboard any longer, though, the directory still exists in the plugins folder on the server.