Accept msgpack 0.4.7 or later

"rake spec" passes with msgpack 0.5.5 on my environment. (Debian
GNU/Linux amd64 sid) So it seems that fluent-logger can accept 0.5.x
series.
Merge pull request #9 from kou/accept-msgpack-0.5-series

Accept msgpack 0.4.7 or later