Reverts a14d2873ca488f430a2fba810740b525b9e8491f :

which states:
"Update the included COPYING file to the GPLv3. As stated in the README, the code is still 'GPL version 2 or later' but we want to encourage distribution under the GPLv3, so we include that version of the license for reference."
This fork has no plans to exercise the "later version" option.