安装ROS出现如下错误：
sudo rosdep init
ERROR: cannot download default sources list from:
https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/sources.list.d/20-default.list
Website may be down.
解决方案：
具体方法见：
https://blog.csdn.net/nanianwochengshui/article/details/105702188