Fix slow_query_log warning for MySQL 5.6.

This commit prevents the following warning from occurring when starting
up MySQL 5.6, etc:

  [Warning] option 'slow_query_log': boolean value '/var/log/mysql/slow.log'
  wasn't recognized. Set to OFF.

This change was taken from a comment to the following pull request:

https://github.com/opscode-cookbooks/mysql/pull/81

Also see the following opscode ticket:

http://tickets.opscode.com/browse/COOK-2681