(use-package simpson-ruby-version
:commands simpson-ruby-version
:config (progn
(setq simpson-ruby-default-gem-path "/usr/local/bin/gem")
(setq simpson-ruby-rubies-path "/usr/local/rubies/")
(setq simpson-ruby-gem-path "~/.gem/ruby/")))
M-x simpson-ruby-version
whenever you need to change the environment to a new Ruby version.