[rubygems] You don't have write permissions for the /var/lib/gems/2.3.0 directory

Building on derek's answer above, it is generally not recommended to use the system provided Ruby instance for your own development work, as system tools might depend on the particular version or location of the Ruby install. Similar to this answer for Mac OSX, you will want to follow derek's instructions on using something like rbenv (RVM is a similar alternative) to install your own Ruby instance.

However, there is no need to uninstall the system version of Ruby, the rbenv installation instructions provide a mechanism to make sure that the instance of Ruby available in your shell is the rbenv instance, not the system instance. This is the

echo 'eval "$(rbenv init -)"' >> ~/.bashrc

line in derek's answer.