解决conda UnavailableInvalidChannel: The channel is not accessible or is invalid.

解决方法：在conda里一行行键入以下命令即可：

conda config --remove channels https://mirrors.tuna.tsinghua.edu.cn/anaconda/pkgs/free/
conda config --add channels https://repo.continuum.io/pkgs/free/ 
conda config --add channels https://repo.continuum.io/pkgs/main/ 
conda config --set show_channel_urls yes