点不会更新到最新版本

Question

当我运行pip install -U pip时，它不会安装新版本。

drahcir@Altair:~/dev$ pip --version
pip 1.5.6 from /usr/lib/python2.7/dist-packages (python 2.7)
drahcir@Altair:~/dev$ pip install -U pip
Downloading/unpacking pip
  Downloading pip-8.1.2-py2.py3-none-any.whl (1.2MB): 1.2MB downloaded
Installing collected packages: pip
Successfully installed pip
Cleaning up...
drahcir@Altair:~/dev$ pip --version
pip 1.5.6 from /usr/lib/python2.7/dist-packages (python 2.7)

即使它说下载了pip 8.1.2，pip版本仍然说1.5.6

如何获得pip升级到8.1.2？