update msgpack 0.5.8

[crow-misia]

Hello.

I updated to 0.5.8 msgpack that depends.

And, append "gcc_atomic.cpp" in compile targets.

Thanks.

[godsflaw]

I will look at this soon (hopefully this week). I have to make the time to update this, test it, and then update the other node msgpack repo and npm.

[hden]

@godsflaw any update on this?

[greglearns]

+1 (need this)

[crow-misia]

Thanks!