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.

See the documentation, including about the versions:
http://dev.mysql.com/doc/refman/5.6/en/replication-options-binary-log.html#sysvar_binlog_checksum

Thanks for the awesome package!