Update submodule sitl_gazebo to latest Fri Sep 14 12:42:47 UTC 2018

    - sitl_gazebo in PX4/Firmware (031885f9a90e70cd8d8aa22c774dfca1bd0c8dfb): https://github.com/PX4/sitl_gazebo/commit/821c55ca714d7acf93d8ed96af71237b8437e488
    - sitl_gazebo current upstream: https://github.com/PX4/sitl_gazebo/commit/9d512af0075659f0ace91ec0e2f8c8a170f911fb
    - Changes: https://github.com/PX4/sitl_gazebo/compare/821c55ca714d7acf93d8ed96af71237b8437e488...9d512af0075659f0ace91ec0e2f8c8a170f911fb

    9d512af 2018-09-12 Lorenz Meier - Update gazebo_wind_plugin.cpp
46d0be1 2018-09-12 Lorenz Meier - Update Wind.proto
14f09ff 2018-09-03 Elia Tarasov - fix compilation for gazebo version equal or greater 9
de86d7b 2018-08-31 Elia Tarasov - use template class vector3d from ignition::math
f3cba29 2018-08-31 Elia Tarasov - enable wind plugin

sitl_gazebo @ 9d512af0

-Subproject commit 821c55ca714d7acf93d8ed96af71237b8437e488
+Subproject commit 9d512af0075659f0ace91ec0e2f8c8a170f911fb