Follow REP 117: +Inf for max range (no reading)

REP 117 (http://www.ros.org/reps/rep-0117.html) establishes that the laser scan max range (no reading) must be set to +Inf.

Note that the navigation stack and other lasers, like the hokuyo node (https://github.com/ros-drivers/hokuyo_node), follow the REP 117.