Fix bumpers in sdf

Author: javizqh

Worlds/roomba_1_house_harmonic.world

@@ -11,6 +11,7 @@
     <plugin filename="gz-sim-sensors-system" name="gz::sim::systems::Sensors">
       <render_engine>ogre2</render_engine>
     </plugin>
+    <plugin name="gz::sim::systems::Contact" filename="gz-sim-contact-system"></plugin>
 
     <scene>
       <ambient>0.4 0.4 0.4 1</ambient>