cap Pillow < 4.2.0
cap Pillow < 4.2.0

There is an API change in Pillow 4.2.0 that breaks our watermarking
step.

Change-Id: I8a8c9d87a7165521c8d86167657b25dc51d5b610
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals