problem with limit_conn

hi guys ,
I am using nginx version 1.0.10 on linux centos 6.5 . I want to add the following to lines to my nginx config file:
limit_conn_zone $binary_remote_addr zone=conn_limit_per_ip:20m;
limit_conn conn_limit_per_ip 3;

But after that when I try to restart nginx I get the following error :
Starting nginx: nginx: [emerg] unknown directive "limit_conn_zone" in /etc/nginx/nginx.conf:31
[FAILED]

what should I do to solve this ?
thank you