license: upgrade from GPLv2-only to GPLv2-or-later
The parser has been GPLv2+GPLv3 for a long time (from unicorn).
Note: I currently (strongly) prefer the GPLv3.
In case the GPLv4 arrives and I am not alive to approve/review it,
the lesser of evils is have give blanket approval of all future GPL
versions (as published by the FSF). The worse evil is to be stuck
with a license which cannot guarantee the Free-ness of this project.
This unfortunately means the FSF can theoretically come out with
license terms I do not agree with, but the GPLv2 and GPLv3 will
always be an option to all users.