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

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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