Fix SafeConfigParser deprecation warning in cli
    
    Replace SafeConfigParser with ConfigParser as suggested by the warning.
    
        waiverdb/cli.py:187: DeprecationWarning: The SafeConfigParser class
        has been renamed to ConfigParser in Python 3.2. This alias will be
        removed in future versions. Use ConfigParser directly instead.
    
    Signed-off-by: Lukas Holecek <hluk@email.cz>