add 2.4.0-prewview2 definitions

https://www.ruby-lang.org/en/news/2016/09/08/ruby-2-4-0-preview2-released/

share/ruby-build/2.4.0-preview2

+install_package "openssl-1.0.2h" "https://www.openssl.org/source/openssl-1.0.2h.tar.gz#1d4007e53aad94a5b2002fe045ee7bb0b3d98f1a47f8b2bc851dcd1c74332919" mac_openssl --if has_broken_mac_openssl
+install_package "ruby-2.4.0-preview2" "https://cache.ruby-lang.org/pub/ruby/2.4/ruby-2.4.0-preview2.tar.bz2#2224c55b2d87b5c0f08d23a4618e870027dbc1cffbfb4a05efd19eac4ff4cf1d" ldflags_dirs standard verify_openssl