Adding a `runInSandbox` overload taking `Callable` for the next version. Or, today:

```
final AtomicReference<Object> ref = new AtomicReference<Object>();
GroovySandbox.runInSandbox(new Runnable() {
    @Override public void run() {
        ref.set(groovyShell.evaluate(script);
    }
}, Whitelist.all());
evaluate = ref.get();
```