yum命令升级的时候，报出这个错误。
There was a problem importing one of the Python modules
required to run yum. The error leading to this problem was:
No module named yum
Please install a package which provides this module, or
verify that the module is installed correctly.
It's possible that the above module doesn't match the
current version of Python, which is:
2.7.5 (default, Sep 3 2013, 23:16:48)
[GCC 4.6.3 20120306 (Red Hat 4.6.3-2)]解决方法：
sudo vim /usr/bin/yum
#修个python所在的路径，比如
#/usr/local/bin/python2.6
