Fix Python tarfile usage for Python 2.6 (https://travis-ci.org/openSUSE/obs-service-tar_scm/jobs/30493348)

The 'filter' parameter was introduced with Python 2.7 so we need to fall-back
to the deprecated 'exclude' parameter.

Signed-off-by: Jan Blunck <jblunck@infradead.org>