Update submodule sitl_gazebo to latest Tue Jun 30 14:39:57 UTC 2020 (#…

…15248)

- sitl_gazebo in PX4/Firmware (38588f0): PX4/PX4-SITL_gazebo-classic@309f7b0
    - sitl_gazebo current upstream: PX4/PX4-SITL_gazebo-classic@1af7e29
    - Changes: PX4/PX4-SITL_gazebo-classic@309f7b0...1af7e29

    1af7e29 2020-06-25 JaeyoungLim - Do not validate r1_rover
5e8ddfb 2020-06-25 Benjamin Perseghetti - fixes for R1 SDF PX4/PX4-SITL_gazebo-classic#533 (#534)
726c066 2020-06-23 Supakorn Suttiruang - Remove the incorrect force generated by moment (#532)

Co-authored-by: PX4 BuildBot <bot@px4.io>