Please make sure the:
ls -l /opt/bitnami/ruby/lib/ruby/site_ruby/2.1.0/x86_64-linux/
say that the folder belongs to you, not to root:root
. If the rights are for root user, you can't create the folder, so to fix it just do:
sudo chown bitnami:bitnami /home/bitnami/. -R
I strongly advice you to use rvm and gemset in order to not using the root operations. Do as follows: