错误如下：
Traceback (most recent call last): File "/usr/bin/pip",line 5,in <module> from pkg_resources import load_entry_point File "/usr/local/lib/python2.7/dist-packages/pkg_resources/__init__.py",line 70,in <module> import packaging.version ImportError: No module named packaging.version
解决方法：
sudo apt-get purge python-pip
wget https://bootstrap.pypa.io/get-pip.py
python get-pip.py
参考http://askubuntu.com/questions/561377/pip-wont-run-throws-errors-instead
