build: log pyflakes version

This may be useful in order to trace inconsistent errors in builds.

Change-Id: I86d82b5fda7e7f2900b62bcf7c3654488f83f770
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28447
Reviewed-by: David Caro <dcaroest@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>