MAINTAINERS: update Serge Hallyn's email address

Author: Serge Hallyn <serge.hallyn@ubuntu.com>
Date:   Thu May 19 23:37:05 2016 -0500

    Update email address

    My canonical.com email address will soon stop working.

Signed-off-by: Serge Hallyn <serge@hallyn.com>

Signed-off-by: James Morris <james.l.morris@oracle.com>