hector_gazebo: removed warnings due to deprecated sdf API calls

Conflicts:

	hector_gazebo_plugins/include/hector_gazebo_plugins/update_timer.h
	hector_gazebo_plugins/src/gazebo_ros_imu.cpp
hector_gazebo_plugins: fixed a bug in UpdateTimer class when updateRate and updatePeriod parameters are unset
Merge remote-tracking branch 'origin/groovy-devel' into hydro-devel