How to add RaySensor plugin to the world file?

According to modification of this [tutorial](http://gazebosim.org/tutorials?tut=contact_sensor) I managed to create a plugin for a ray sensor. I also added the line `` to the .world file directly bellow `` just like in the tutorial. However, when I open the world in Gazebo, the plugin doesn't appear there. The path to the plugin is correct, as I am able to use model plugins. I'm using Gazebo 9 on Ubuntu 18.4.1.