Allow MQTT adapter to depend on snapshot version of vertx-mqtt.
Allow MQTT adapter to depend on snapshot version of vertx-mqtt.

There currently doesn't exist a release version of vertx-mqtt. However,
we want to include the MQTT adapter in our milestone releases. Thus, we
need to skip the corresponding enforcer rule.
dejanb/hono,kinbod/hono,dejanb/hono,dejanb/hono,kinbod/hono,kinbod/hono