license: allow all future versions of the GNU GPL

There is currently no GPLv4, so this change has no effect at the
moment.

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
in the future.

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.