clarify the license

[drzraf]

The file LICENCE is a copy of the GNU GENERAL PUBLIC LICENSE v2.
But source code headers state:

- that is bundled with this package in the file LICENSE.txt```
  While I would be personally in favor of the GPL (ideally and in order : AGPLv3, then GPLv3+ then GPLv2+), I don't know:
  - if it's even possible given code and API of PrestaShop itself
  - if GPLv2 was really intended as the license for the module