fix 1 of the 2 Nan::Callback::Call deprecation warnings

Looks like the Nan::Callback::Call() API deprecated bare calls that
don't pass a new Nan::AsyncResource().  Luckily, in this case we were
already subclassing Nan::AsyncWorker, which has a member named
`async_resource` so all we have to do is explicitly pass this along.

I wonder if Nan should just do this for me? *hmm*

Bug #209

src/node_nanomsg.cc

@@ -230,7 +230,7 @@ class NanomsgDeviceWorker : public Nan::AsyncWorker {
 
     Local<Value> argv[] = { Nan::New<Number>(err) };
 
-    callback->Call(1, argv);
+    callback->Call(1, argv, async_resource);
   };
 
 private: