The contents of my launch 
turtlebot_world.launch----
<launch>
  <arg name="gui"        default="true"/>
  <arg name="world_file" default="$(env TURTLEBOT_GAZEBO_WORLD_FILE)"/>
  <arg name="base"       value="$(optenv TURTLEBOT_BASE kobuki)"/> 
  <arg name="battery"    value="$(optenv TURTLEBOT_BATTERY /proc/acpi/battery/BAT0)"/>  
  <arg name="stacks"     value="$(optenv TURTLEBOT_STACKS hexagons)"/>  
  <arg name="3d_sensor"  value="$(optenv TURTLEBOT_3D_SENSOR kinect)"/>  
  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="use_sim_time" value="true"/>
    <arg name="debug" value="false"/>
    <arg name="gui" value="$(arg gui)" />
    <arg name="world_name" value="$(arg world_file)"/>
  </include>
  <include file="$(find turtlebot_gazebo)/launch/includes/$(arg base).launch.xml">
    <arg name="base" value="$(arg base)"/>
    <arg name="stacks" value="$(arg stacks)"/>
    <arg name="3d_sensor" value="$(arg 3d_sensor)"/>
  </include>
  <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher">    
  </node>  
  <node pkg="nodelet" type="nodelet" name="laserscan_nodelet_manager" args="manager"/>
  <node pkg="nodelet" type="nodelet" name="depthimage_to_laserscan"
        args="load depthimage_to_laserscan/DepthImageToLaserScanNodelet laserscan_nodelet_manager">    
            <remap from="image" to="/camera/depth/image_raw"/>
    <remap from="scan" to="/scan"/>
  </node>
</launch>