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At first it's necessary to connect the joint with actuator:

<transmission name="joint_back_right_wheel_trans" type="SimpleTransmission">
     <actuator name="back_right_wheel_motor"/>
     <joint name="wheel_to_back_right_wheelholder"/>
         <mechanicalReduction>1</mechanicalReduction>
         <motorTorqueConstant>1</motorTorqueConstant>
        </transmission>

Then add these lines to your robot description file:

<gazebo>
   <controller:gazebo_ros_controller_manager name="gazebo_ros_controller_manager" plugin="libgazebo_ros_controller_manager.so">
      <alwaysOn>true</alwaysOn>
      <updateRate>1000.0</updateRate>
      <interface:audio name="gazebo_ros_controller_manager_dummy_iface" />
    </controller:gazebo_ros_controller_manager>
  </gazebo>

It should work then.