# sudo apt-get install Python-software-properties
# sudo apt-add-repository ppa:brightbox/ruby-ng
# sudo apt-get update
# sudo apt-get install ruby2.1 ruby2.1-dev

解决方案：

apt-get install python-software-properties
apt-get install software-properties-common

2、当运行apt-get update后出现如下错误时：

E: Some index files failed to download, they have been ignored, or old ones used instead.

解决方案：

rm /var/lib/apt/lists/partial/*