Update platforms in the kitchen config [skip ci]

.kitchen.yml

@@ -5,7 +5,7 @@ provisioner:
   name: chef_zero
 
 platforms:
-  - name: centos-6.8
+  - name: centos-6.9
   - name: centos-7.3
   - name: debian-7.11
     run_list: apt::default