changed shape_server code to run with the hydro mversion of turtlesim ("...

Hi,

I just recognized that the hydro version of the shape_server does not work.
In Hydro turtlesim changed the navigation message type from "turtle_actionlib/Velocity.h" to "geometry_msgs/Twist.h" and the corresponding topic from "command_velocity" to "cmd_vel"

This fix will solve the problem

I have no idea why noone had this problem before