Directory is not a symbol link created by iRedMail

==== Required information ====
- iRedMail version: iRedMail-0.9.1-beta1
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MariaDB
- Web server (Apache or Nginx): nginx
- Linux/BSD distribution name and version: Ubuntu 14.04 LTS
- Related log if you're reporting an issue:
====

Running bash upgrade_iredadmin.sh failed with error

# bash upgrade_iredadmin.sh 
* Detected Linux/BSD distribution: UBUNTU
<<< ERROR >>> Directory is not a symbol link created by iRedMail. Exit