Update submodule sitl_gazebo to latest Mon Apr 2 17:13:58 UTC 2018

 - sitl_gazebo in PX4/Firmware (d05bc28): PX4/PX4-SITL_gazebo-classic@7c5a7bb
 - sitl_gazebo current upstream: PX4/PX4-SITL_gazebo-classic@28921bc
 - Changes: PX4/PX4-SITL_gazebo-classic@7c5a7bb...28921bc

28921bc 2018-03-27 amy wagoner - Added fpv_cam.sdf as an exception.
c84d79c 2018-03-26 amy wagoner - removed unexpected element 'torsional'
4d777a7 2018-03-26 amy wagoner - Removed 'pose frame'
771265b 2018-03-26 amy wagoner - Added iris fpv world.
016a612 2018-03-26 amy wagoner - Added generic camera model and iris fpv model.