Unnecessary dependancy on PDO mysql

[tobarja]

This commit: a1304ad uses a constant that is only defined on machines with the PDO mysql extension compiled in.

Commenting out the MYSQL_ATTR_INIT_COMMAND line fixes it in my vagrant image.

If this setting is necessary, perhaps a method like this one should be used: http://php.net/manual/en/pdo.constants.php#pdo.constants.attr-driver-name