add Ruby 2.4 and maintain passing versions
add Ruby 2.4 and maintain passing versions

ojundt/msgpack-ruby,cosmo0920/msgpack-ruby,cosmo0920/msgpack-ruby,ojundt/msgpack-ruby,ojundt/msgpack-ruby,msgpack/msgpack-ruby,ojundt/msgpack-ruby,cosmo0920/msgpack-ruby,cosmo0920/msgpack-ruby,msgpack/msgpack-ruby,msgpack/msgpack-ruby