Update submodule sitl_gazebo to latest Thu May 31 01:26:50 UTC 2018

    - sitl_gazebo in PX4/Firmware (1ee08da): PX4/PX4-SITL_gazebo-classic@651ca35
    - sitl_gazebo current upstream: PX4/PX4-SITL_gazebo-classic@371e7c3
    - Changes: PX4/PX4-SITL_gazebo-classic@651ca35...371e7c3

    371e7c3 2018-04-10 korotkoves - udp port for each model instance from options file
376258d 2018-04-17 korotkoves - change file format to use common and model specific parameters
b4b7245 2018-01-17 korotkoves - function to load parameters for each model from xml file