You are using pip version 8.1.1, however version 18.0 is available.

问题：

You are using pip version 8.1.1, however version 18.0 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.

解决方法：

wget https://bootstrap.pypa.io/get-pip.py

sudo python get-pip.py

即可