Update submodule sitl_gazebo to latest Wed Dec 26 12:37:45 UTC 2018

    - sitl_gazebo in PX4/Firmware (1f9b8333): https://github.com/PX4/sitl_gazebo/commit/c90e8ad2b27b95dc0b4c45b346509477c5fa593d
    - sitl_gazebo current upstream: https://github.com/PX4/sitl_gazebo/commit/e945745021a74ae45695a624dfc114a5c7bbd6bc
    - Changes: https://github.com/PX4/sitl_gazebo/compare/c90e8ad2b27b95dc0b4c45b346509477c5fa593d...e945745021a74ae45695a624dfc114a5c7bbd6bc

    e945745 2018-12-26 Julian Oes - worlds: fix rate for tailsitter and plane
46a3d27 2018-12-14 Jörg Schmidt - Fix bug in groundtruth altitude calculation

sitl_gazebo @ e9457450

-Subproject commit c90e8ad2b27b95dc0b4c45b346509477c5fa593d
+Subproject commit e945745021a74ae45695a624dfc114a5c7bbd6bc