hector_gazebo_plugins: replaced deprecated ConnectWorldUpdateStart/DisconnectWorldUpdateStart events by ConnectWorldUpdateBegin/DisconnectWorldUpdateBegin for Gazebo 1.5
hector_gazebo_plugins: fixed UpdateTimer class

Usage of the fmod() function to check if the current sim time hits a period+-(step time/2) leads to numerically instable results.
bumped version to 0.2.4