Expose Ivy's circular dependency check via UpdateOptions

See http://ant.apache.org/ivy/history/2.3.0/concept.html#circular

By default circular dependencies are warned, but they do not halt the dependency resolution. Using the following setting, circular dependencies can be treated as an error.

```scala
updateOptions := updateOptions.value.withCircularDependencyLevel(CircularDependencyLevel.Error)
```

I guess I should seal that sucker.