Add Ruby 2.4.0-rc1 definition

See the release announcement: https://www.ruby-lang.org/en/news/2016/12/11/ruby-2-4-0-rc1-released/

share/ruby-build/2.4.0-rc1

+install_package "openssl-1.0.2j" "https://www.openssl.org/source/openssl-1.0.2j.tar.gz#e7aff292be21c259c6af26469c7a9b3ba26e9abaaffd325e3dccc9785256c431" mac_openssl --if has_broken_mac_openssl
+install_package "ruby-2.4.0-rc1" "https://cache.ruby-lang.org/pub/ruby/2.4/ruby-2.4.0-rc1.tar.bz2#3b156b20f9df0dd62cbeeb8e57e66ea872d2a5b55fabdef1889650122bcc2ea7" ldflags_dirs standard verify_openssl