Upgrade license to GPLv3

[skangas]

Please consider upgrading the license to GPLv3.

Reasons why:
https://lists.gnu.org/archive/html/emacs-devel/2017-07/msg01069.html
https://www.gnu.org/licenses/rms-why-gplv3.html

The main file of this project says:

;; This program 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
;; (at your option) any later version.

So this upgrade should be unproblematic.

Thanks!