Update install/CentOS_6.md

Updated gitlab-centos script to 4-0-stable to fix unicorn and sidekiq startup errors.

Author: naillizard

install/CentOS_6.md

@@ -466,7 +466,7 @@ Download the init script (will be /etc/init.d/gitlab)
 
 *logged in as root*
 
-    curl https://raw.github.com/gitlabhq/gitlab-recipes/master/init.d/gitlab-centos > /etc/init.d/gitlab
+    curl https://raw.github.com/gitlabhq/gitlab-recipes/4-0-stable/init.d/gitlab-centos > /etc/init.d/gitlab
     chmod +x /etc/init.d/gitlab
     chkconfig --add gitlab