error: ‘class gazebo::physics::World’ has no member named ‘LightCount’

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I am using Gazebo 7 with ROS kinetic on Ubuntu 16.04. I wrote a small gazebo plugin with gets loaded when I load a world in gazebo. However, when I try to use LightCount API, I get the compile error saying "error: ‘class gazebo::physics::World’ has no member named ‘LightCount’".
Here is the link that says that this API is supported http://osrf-distributions.s3.amazonaw...

Here is the code snippet that is giving me error
   // Called by the world update start event
    public: void OnUpdate(const common::UpdateInfo & /*_info*/)
    {
       physics::WorldPtr w = this->model->GetWorld();
       unsigned int i = w->LightCount();
       printf("Light count = %d\n", i); 
    }