https://www.youtube.com/watch?v=cAOYjAFYpGg
rbenv install -l rbenv install 2.3.1 #=> Downloading ruby-2.3.1.tar.bz2... #=> -> https://cache.ruby-lang.org/pub/ruby/2.3/ruby-2.3.1.tar.bz2 #=> Installing ruby-2.3.1.. rbenv install 2.4.0-preview1