Upgrade cool.io Dependency Version

[Olart]

Hi,

You should upgrade the cool.io dependency version to something newer than 1.2.4. Cool.io 1.2.4 has some compatibility issues with Developers tools, preinstalled on OS X Yosemite and El Capitan (Command line tools v. 7.0 and above).

I've spent several hours, determining the reason, why installing Cool.io dependency was failing.

[suma]

Upgraded cool.io via #5, and msgpack-rpc v0.5.4 released. Please try it.

[prdn]

Another upgrade of cool.io might be good