[Migration] Fixed script. Column name and table name were swapped.

application/migrations/092_add_service_username.php

@@ -20,7 +20,7 @@ class Migration_add_service_username extends CI_Migration {
                 )
             );
 
-            if (!$this->db->field_exists('thirdparty_logins', 'service_username')) {
+            if (!$this->db->field_exists('service_username', 'thirdparty_logins')) {
                 $this->dbforge->add_column('thirdparty_logins', $fields);
             }