Update submodule sitl_gazebo to latest Wed Apr 3 12:38:08 UTC 2019

    - sitl_gazebo in PX4/Firmware (d36c0e13): https://github.com/PX4/sitl_gazebo/commit/2f0070ec6c29c34b9bae10ac03224775627c338a
    - sitl_gazebo current upstream: https://github.com/PX4/sitl_gazebo/commit/e0dfe7ea45c849e676c201b7428bf8df975d4790
    - Changes: https://github.com/PX4/sitl_gazebo/compare/2f0070ec6c29c34b9bae10ac03224775627c338a...e0dfe7ea45c849e676c201b7428bf8df975d4790

    e0dfe7e 2019-03-11 Jannik Beyerstedt - Fix compiler error with openCV (C++11) on macOS