Log Message:
Update ruby-msgpack to 0.7.4.

2016-01-08 version 0.7.4:

* Improved compatibility of Packer between CRuby and JRuby about argument
  IO-ish object values.

2016-01-07 version 0.7.3:

* Add Packer#write_float32 method to pack floating point numbers into FLOAT
  of messagepack.