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

 - sitl_gazebo in PX4/Firmware (d05bc285): https://github.com/PX4/sitl_gazebo/commit/7c5a7bb226f503ce7596c2067dac828cf07abcaa
 - sitl_gazebo current upstream: https://github.com/PX4/sitl_gazebo/commit/28921bca9df9a212675b8e888c926d1ec6f6b28e
 - Changes: https://github.com/PX4/sitl_gazebo/compare/7c5a7bb226f503ce7596c2067dac828cf07abcaa...28921bca9df9a212675b8e888c926d1ec6f6b28e

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.

sitl_gazebo @ 28921bca

-Subproject commit 7c5a7bb226f503ce7596c2067dac828cf07abcaa
+Subproject commit 28921bca9df9a212675b8e888c926d1ec6f6b28e