Update submodule sitl_gazebo to latest Sat Jun 23 01:30:51 UTC 2018

    - sitl_gazebo in PX4/Firmware (1dd67e77): https://github.com/PX4/sitl_gazebo/commit/b5a92095bfb1b6a218a80f58ad9fe09c89ef4598
    - sitl_gazebo current upstream: https://github.com/PX4/sitl_gazebo/commit/3c7d18d0a52657106a3a7190c54006ed6b442071
    - Changes: https://github.com/PX4/sitl_gazebo/compare/b5a92095bfb1b6a218a80f58ad9fe09c89ef4598...3c7d18d0a52657106a3a7190c54006ed6b442071

    3c7d18d 2018-06-19 TSC21 - travis: add -DBUILD_ROS_INTERFACE=ON option to catkin build
e89cf8f 2018-06-17 TSC21 - travis: use catkin build tools
f7a1c4b 2018-06-19 Elia Tarasov - remove unnecessary mavros libs and duplicate CMake message
c495c57 2018-06-19 Elia Tarasov - remove unnecessary dependecies
5c861a5 2018-06-19 Elia Tarasov - remove executable node
7d69268 2018-06-19 Elia Tarasov - remove redundant includes
6ba0eeb 2018-06-19 Elia Tarasov - remove redundant ros packages
9c69a95 2018-06-18 Elia Tarasov - modify target node name and add needed libs
147cb56 2018-06-17 elia - move class member function definitions to implementation file
dc24fcc 2018-06-17 elia - add undefined class varibales

sitl_gazebo @ 3c7d18d0

-Subproject commit b5a92095bfb1b6a218a80f58ad9fe09c89ef4598
+Subproject commit 3c7d18d0a52657106a3a7190c54006ed6b442071