This is really just a handy OSX development note.
brew update brew upgrade ruby-build
Then list the available ruby versions:
rbenv install --list
And finally, if you want to install a new version, pick one from the list and run the installation command:
rbenv install 2.3.1
Now we are in business!
I often find myself reverting to the source or linux install route to upgrade ruby versions via ruby-build. So, hopefully this serves as a useful memory jog for a few others.