[docker] use libgazebo9 from ubuntu 18.04

docker/ci/Dockerfile

@@ -1,8 +1,6 @@
 FROM paparazziuav/pprz-dep
 LABEL maintainer="felix.ruess@gmail.com"
 
-RUN sh -c 'echo "deb http://packages.osrfoundation.org/gazebo/ubuntu-stable `lsb_release -cs` main" > /etc/apt/sources.list.d/gazebo-stable.list'
-RUN wget http://packages.osrfoundation.org/gazebo.key -O - | apt-key add -
 RUN curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain nightly
 ENV PATH $HOME/.cargo/bin:$PATH
 RUN /bin/bash -c "source $HOME/.cargo/env; \
@@ -11,7 +9,7 @@ RUN /bin/bash -c "source $HOME/.cargo/env; \
 
 RUN DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y \
     g++-arm-linux-gnueabi \
-    libgazebo8-dev \
+    libgazebo9-dev \
     rustc cargo \
     python3-lxml \
     && rm -rf /var/lib/apt/lists/*