Handle binlog_checksum variable missing in MySQL < 5.6.2

Hello,

I have run into an issue using MySQL 5.5.40 on Ubuntu 14.04.1. The MySQL global variable `@@GLOBAL.binlog_checksum` does not exist. With this patch, the process runs properly.

This only seems to be effecting the `master` branch. The published NPM package 0.2.0 seems to work fine. I was evaluating the package by cloning it and encountered this but now that I am using `npm install` the error does not occur.

Thanks for the awesome package!