This is the default behavior of rbenv and what users are expecting
most of the time.
It allows users to have their own set of rubies and gems. It also
prevents losing all rubies when rbenv is updated using Homebrew which
is not true when RBENV_ROOT is set to /usr/local/opt/rbenv.
Fixes gem versions with words (beta, pre) in the version or with
multiple installed versions not being highlighted, and compatibility
with GNU sed (related to robbyrussell/oh-my-zsh#1579).