Update ruby-msgpack to 0.7.1.

2015-11-20 version 0.7.1:

* Fixed bug to pack/unpack ext type objects larger than 256bytes incorrectly.

2015-10-24 version 0.7.0:

* Add extention types support.
* Fix to share almost all test cases between CRuby and JRuby implementations.
* Fixed JRuby implementation to raise UnknownExtTypeError for unregistered ext type ids
  instead to generate MessagePack::ExtensionValue instances.
  (Specify `allow_unknown_ext: true` as unpacker option for v0.6.x behavior.)

(taca)