Clarify that dulwich is GPLv2 *or later*.

setup.py

@@ -13,7 +13,7 @@ setup(name='dulwich',
       version=dulwich_version_string,
       url='http://launchpad.net/dulwich',
       download_url='http://samba.org/~jelmer/dulwich/dulwich-%s.tar.gz' % dulwich_version_string,
-      license='GPL',
+      license='GPLv2 or later',
       author='Jelmer Vernooij',
       author_email='jelmer@samba.org',
       long_description="""