ROS Melodic自带的Gazebo版本过低，建议升级

# 1.首先，查看Gazebo版本(出现的是 gazebo9 及其相关插件)
~$ dpkg -l | grep gazebo
# 2.卸载全部插件
~$ sudo apt-get remove gazebo9 gazebo9-common gazebo9-plugin-base libgazebo9:amd64 libgazebo9-dev:amd64 ros-melodic-gazebo-* 
# 3.配置镜像
~$ sudo sh -c 'echo "deb http://packages.osrfoundation.org/gazebo/ubuntu-stable `lsb_release -cs` main" > /etc/apt/sources.list.d/gazebo-stable.list'
# 4.可以通过下述命令查看文件是否写入正确，会出现deb http://packages.osrfoundation.org/gazebo/ubuntu-stable bionic main
~$ cat /etc/apt/sources.list.d/gazebo-stable.list
# 5.设置key
~$ wget https://packages.osrfoundation.org/gazebo.key -O - | sudo apt-key add -
# 6.更新 the debian database
~$ sudo apt-get update
# 7.安装gazebo11
~$ sudo apt-get install gazebo11
~$ sudo apt-get install libgazebo11-dev
# 8.安装gazebo的ros插件
~$ sudo apt install ros-melodic-gazebo11-*
# 9.检查gazebo是否安装成功
~$ gazebo