Add definition for Ruby 2.3.7

https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-3-7-released/

share/ruby-build/2.3.7

+install_package "openssl-1.0.2n" "https://www.openssl.org/source/openssl-1.0.2n.tar.gz#370babb75f278c39e0c50e8c4e7493bc0f18db6867478341a832a982fd15a8fe" mac_openssl --if has_broken_mac_openssl
+install_package "ruby-2.3.7" "https://cache.ruby-lang.org/pub/ruby/2.3/ruby-2.3.7.tar.bz2#18b12fafaf37d5f6c7139c1b445355aec76baa625a40300598a6c8597fc04d8e" ldflags_dirs standard verify_openssl