Note that: ruby-2.3.0 is not present now when run rvm list known command in rvm 1.26.11.
It may be available in next rvm version. See here for more details.

But you can just run the following directly.

rvm install ruby-2.3.0

And set 2.3.0 as default ruby version

rvm --default use 2.3.0

Copy gems from older version

rvm gemset copy 2.1.5 2.3.0

Get the ruby version and celebrate!

ruby -v
# ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin15]

Try new features in irb

puts 'hi'&.upcase

h = { a: 1, b: 2, c: 3, d: 4 }
[ :a, :c, :d, :b ].map(&h)