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

Try using chown -R on the var/lib/gems directory, assigning ownership to the user [rubyusername] in this example, the user that will be installing and developing with gems.

 # chown -R rubyusername:rubyusername /var/lib/gems 

This recursively changes everything under the gems directory. For extra security on multi-user systems, you can also create a group, rather than chowning the individual rubyusername, and add users to that group.