Possible fix for issue with start_div and end_div in Moodle 2.4.6

When testing this latest of the customsql report with version 2.4.6 of Moodle, I was presented with an empty page where I'd normally have seen all my ad-hoc queries.  Imagine my surprise!  

In reviewing my PHP logs, I see the following:

PHP Fatal error:  Call to undefined method html_writer::start_div() in /root/path/to/moodle/report/customsql/index.php on line 60

Running a Grep on the source looking for html_writer::start_div only returns the two instances found in the index.php file for this custom report.  

Replacing the start_div with start_tag and end_div with end_tag appears to have corrected it on my dev instance and I wanted to share.  

FWIW, I really like addition of categories!  Thanks for your hard work!  If we ever meet at a Moot, beers on me (:beers:).