Move AWS integration adapters to Spring Integration 4.0

No, `milestone` is OK: it includes `release` as well. The `snapshot` is bad.
But we can rely on some Milestone or RC for some project someday.
So, the repo should be:
```
maven { url 'http://repo.spring.io/libs-milestone' }
```

BTW, would be better to upgrade to Gradle `1.12` - `1.6` is enough old...