mom: Require latest mom

Latest mom build uses strings for the balloon info,
needed for proper ballooning integration.

This patch will make vdsm depending on the latest mom
supporting the strings api.

Change-Id: Ia2aebf588f823ee986bd20bbf47bace176b4cd21
Signed-off-by: Doron Fediuck <dfediuck@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17519
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>