Update submodule sitl_gazebo to latest Mon Oct 15 08:37:19 EDT 2018

    - sitl_gazebo in PX4/Firmware (31b110c2): https://github.com/PX4/sitl_gazebo/commit/70ca64654061fb7a4279846d9da1ad3512cfc6ba
    - sitl_gazebo current upstream: https://github.com/PX4/sitl_gazebo/commit/d1f0bd4367d1cab01c772bb593a3f750b2f7dce2
    - Changes: https://github.com/PX4/sitl_gazebo/compare/70ca64654061fb7a4279846d9da1ad3512cfc6ba...d1f0bd4367d1cab01c772bb593a3f750b2f7dce2

    d1f0bd4 2018-10-10 Elia Tarasov - remove redundant check for gazebo major version greater than 7
a796798 2018-10-10 Elia Tarasov - change gazebo version for irlock plugin
83cadac 2018-10-10 Elia Tarasov - fix compile error for sonar plugin
aac3d20 2018-10-10 Elia Tarasov - fix compile error for lidar plugin
5a497ad 2018-10-10 Elia Tarasov - fix gps delay computation
3f0a769 2018-10-10 Elia Tarasov - fill timestamp inside plugin callback
779954c 2018-10-10 Elia Tarasov - send and recv sitl_gazebo message timestamp as unified type
46c4b29 2018-10-10 Elia Tarasov - proto: change timestamp name and type for consistency