Update check_bodhi_version to check for >= 6.0.0
This loses the logic for handling an exception if we're under
2.0 or bodhi-client isn't installed at all, but ehhh, that seems
unnecessary. It's pretty unlikely anyone could manage to have
< 2.0 at this point.
Merges: https://pagure.io/fedpkg/pull-request/560
Signed-off-by: Adam Williamson <awilliam@redhat.com>