Fix deprecated callback

The following warning.

```
../src/worker.cc: In member function ‘virtual void
CheckSpellingWorker::HandleOKCallback()’:
../src/worker.cc:44:25: warning: ‘v8::Local<v8::Value>
Nan::Callback::Call(int, v8::Local<v8::Value>*) const’ is deprecated
[-Wdeprecated-declarations]
   callback->Call(2, argv);
```

src/worker.cc

@@ -41,5 +41,6 @@ void CheckSpellingWorker::HandleOKCallback() {
   }
 
   Local<Value> argv[] = { Nan::Null(), result };
-  callback->Call(2, argv);
+  Nan::AsyncResource resource("CheckSpellingWorker::HandleOKCallback");
+  callback->Call(2, argv, &resource);
 }