Update submodule sitl_gazebo to latest Wed Jun 20 21:26:24 EDT 2018

    - sitl_gazebo in PX4/Firmware (7a82c77): PX4/PX4-SITL_gazebo-classic@b5a9209
    - sitl_gazebo current upstream: PX4/PX4-SITL_gazebo-classic@3c7d18d
    - Changes: PX4/PX4-SITL_gazebo-classic@b5a9209...3c7d18d

    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