DatabaseMysqlBase: Use VERSION() rather than server_info

I noticed enwiki's Special:Version page was reporting the version
of MariaDB 10 as "5.5.5-10.0.11-MariaDB-log" rather than just
"10.0.11-MariaDB-log". This change should fix that.

Change-Id: I6bf7e27e88014f70594b33d089636b09b6c97527
Merge "DatabaseMysqlBase: Use VERSION() rather than server_info"