Install binary as bin/ruby as well as bin/jruby

Most Ruby scripts that use a shebang will want ruby to be the command name, but even after installing jruby-launcher our bin/ruby is still a bash script. We should make bin/ruby also point at the executable, either by copying or linking.

Note this also applies to JRuby on Windows; the binary executable should be used for both bin/jruby and bin/ruby.