Update submodule sitl_gazebo to latest Wed Oct 3 18:37:46 UTC 2018 (#10643)

    - sitl_gazebo in PX4/Firmware (6c34407d): https://github.com/PX4/sitl_gazebo/commit/9d512af0075659f0ace91ec0e2f8c8a170f911fb
    - sitl_gazebo current upstream: https://github.com/PX4/sitl_gazebo/commit/1264b74493cd2f362d852530bcc7423a013f2151
    - Changes: https://github.com/PX4/sitl_gazebo/compare/9d512af0075659f0ace91ec0e2f8c8a170f911fb...1264b74493cd2f362d852530bcc7423a013f2151

1264b74 2018-09-30 TSC21 - gazebo_motor_failure_plugin: clean up and fix for Gazebo 9
77141e7 2018-09-29 TSC21 - Travis: add naming for jobs within the build matrix
e46f82a 2018-09-29 TSC21 - add new 18.04 Melodic build
5b93919 2018-09-29 TSC21 - Travis: update ROS docker container