Add definition for Ruby 2.5.1

https://www.ruby-lang.org/en/news/2018/03/28/ruby-2-5-1-released/

share/ruby-build/2.5.1

+install_package "openssl-1.1.0g" "https://www.openssl.org/source/openssl-1.1.0g.tar.gz#de4d501267da39310905cb6dc8c6121f7a2cad45a7707f76df828fe1b85073af"  mac_openssl --if has_broken_mac_openssl
+install_package "ruby-2.5.1" "https://cache.ruby-lang.org/pub/ruby/2.5/ruby-2.5.1.tar.bz2#0f5d20f012baca865381a055e73f22db814615fee3c68083182cb78a4b3b30cb" ldflags_dirs standard verify_openssl