Since the license terms in the changelog allow it, bump to GPL 3.

debian/copyright

@@ -3,10 +3,10 @@ Format: http://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
 Files: *
 Copyright: 2014 Rohan Garg <shadeslayer@systems>
            2017-2018 Lubuntu Developers <lubuntu-devel@lists.ubuntu.com>
-License: GPL-2+
+License: GPL-3+
  This package is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
- the Free Software Foundation; either version 2 of the License, or
+ the Free Software Foundation; either version 3 of the License, or
  (at your option) any later version.
  .
  This package is distributed in the hope that it will be useful,
@@ -18,4 +18,4 @@ License: GPL-2+
  along with this program. If not, see <http://www.gnu.org/licenses/>
  .
  On Debian systems, the complete text of the GNU General
- Public License version 2 can be found in "/usr/share/common-licenses/GPL-2".
+ Public License version 3 can be found in "/usr/share/common-licenses/GPL-3".