rbenvをhomebrewでインストールしています。
$ echo "install_package "openssl-1.1.0j" "https://www.openssl.org/source/openssl-1.1.0j.tar.gz#31bec6c203ce1a8e93d5994f4ed304c63ccf07676118b6634edded12ad1b3246" mac_openssl --if has_broken_mac_openssl\n
install_package "ruby-2.6.0-rc2" "https://cache.ruby-lang.org/pub/ruby/2.6/ruby-2.6.0-rc2.tar.bz2#b3d03e471e3136f43bb948013d4f4974abb63d478e8ff7ec2741b22750a3ec50" ldflags_dirs standard verify_openssl" > /usr
/local/Cellar/ruby-build/20181207/share/ruby-build/2.6.0-rc2
$ rbenv install 2.6.0-rc2