Add rb_hash_clear definition for rubinius (including "st.h" doesn't work well)
Add rb_hash_clear definition for rubinius (including "st.h" doesn't work well)

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