PHP Fatal error: Call to undefined method Piwik\Piwik::isUserIsSuperUser() (2 replies)

I've googled and found a few similar issues/bugs but none with this exact same error so here it goes:

After upgrading to Piwik 2.3 from the last version, everything seems to work fine except that I get this error when clicking on "Settings"

Any ideas?

2014/05/27 05:23:33 [error] 544323#0: *133697 FastCGI sent in stderr: "PHP message: PHP Fatal error:  Call to undefined method Piwik\Piwik::isUserIsSuperUser() in /var/www/clients/client1/web36/web/plugins/SecurityInfo/SecurityInfo.php on line 36" while reading response header from upstream, client: 197.79.34.200, server: analytics.ict-consult.co.za, request: "GET /index.php?module=UsersManager&action=userSettings&idSite=4&period=range&date=previous7 HTTP/1.1", upstream: "fastcgi://unix:/var/lib/php5-fpm/web36.sock:", host: "analytics.ict-consult.co.za", referrer: "http://analytics.ict-consult.co.za/index.php?module=CoreHome&action=index&idSite=4&period=range&date=previous7"