ubuntu下执行sudo apt-get install python-pip 出现错误 Unable to locate package python-pip

ubuntu13.04下执行sudo apt-get install python-pip 出现以下错误：
E: Unable to locate package python-pip

解决办法：
摘自 https://askubuntu.com/questions/672808/sudo-apt-get-install-python-pip-is-failing

python-pip is in the universe repositories, therefore use the steps below:

sudo apt-get install software-properties-common
sudo apt-add-repository universe
sudo apt-get update
sudo apt-get install python-pip