fix(pillar.example): fix `yamllint` error

* https://travis-ci.org/myii/php-formula/builds/594703019#L208-L210

```bash
$ yamllint -s .
./pillar.example
  151:30    error    forbidden implicit octal value "0660"  (octal-values)
```

pillar.example

@@ -148,7 +148,7 @@ php:
             listen: /var/run/php5-fpm-myapp.sock
             listen.owner: www-data
             listen.group: www-data
-            listen.mode: 0660
+            listen.mode: '0660'
             pm: dynamic
             pm.max_children: 5
             pm.start_servers: 2