Update submodule sitl_gazebo to latest Fri Apr 13 20:26:27 CDT 2018

    - sitl_gazebo in PX4/Firmware (4ffa98aa): https://github.com/PX4/sitl_gazebo/commit/28921bca9df9a212675b8e888c926d1ec6f6b28e
    - sitl_gazebo current upstream: https://github.com/PX4/sitl_gazebo/commit/0df46c53b14ef4241b33fb7a74a3cb6f9d83c6dd
    - Changes: https://github.com/PX4/sitl_gazebo/compare/28921bca9df9a212675b8e888c926d1ec6f6b28e...0df46c53b14ef4241b33fb7a74a3cb6f9d83c6dd

    0df46c5 2018-04-13 Tully Foote - Only use catkin env hook command if building with catkin.
38c0d07 2018-04-12 Tully Foote - Add package to GAZEBO_RESOURCE_PATH to make the worlds accessible.
5811488 2018-04-11 Tully Foote - updating OpticalFlow submodule
7c79bd7 2018-04-10 Tully Foote - Add environment hooks to export paths for plugins and resources.

sitl_gazebo @ 0df46c53

-Subproject commit 28921bca9df9a212675b8e888c926d1ec6f6b28e
+Subproject commit 0df46c53b14ef4241b33fb7a74a3cb6f9d83c6dd