Use Nan::Callback::Call instead of v8
Use Nan::Callback::Call instead of v8

So apparently when using

```
Nan::Call(**callback, Nan::GetCurrentContext()->Global(), 1, argv);
```

libuv may not immediately unblock. This causes high latencies in
web servers. Nan actually recommends `callback->Call(...)`, so
we will use this instead.

dsprenkels/sss-node,dsprenkels/sss-node,dsprenkels/sss-node,dsprenkels/sss-node