update msgpack dependency (v0.4.4 -> v0.5.4)

Fluentd with msgpack 0.5.4 works with dramatically less memory than with msgpack 0.4.7.

Memory resource graphs for my personal benchmarks below:

* Fluentd 0.10.32 + ruby 2.0.0-p0 + msgpack 0.4.7

http://cdn-ak.f.st-hatena.com/images/fotolife/t/tagomoris/20130315/20130315173133.png

* Fluentd 0.10.32 + ruby 2.0.0-p0 + msgpack 0.5.4 (09:00 - 12:00)

http://cdn-ak.f.st-hatena.com/images/fotolife/t/tagomoris/20130316/20130316125232.png