Upgrade to 2.3.6 + metaAttribute methods fix

Project has been upgraded to Grails 2.3.6.

The CloudFileInterface has also been updated: method signatures has been changed to `def` as return value instead of `void` for `getMetaAttribute` and `getMetaAttributes`.
But if you prefer, it could be `String` and `Map`.

```groovy
String getMetaAttribute(key)

Map getMetaAttributes()
```