Fixed part of deprecation problem.

`Persistent::Dispose()` will be deprecated and `Persistent::Dispose(Isolate* isolate))` already is, in favor of `Persistent::Reset()`. The future deprecation of `Persistent::Dispose()` will require splitting off more code depending on version, e.g. `NanCallback::~NanCallback()`.

```c++
// TODO(dcarney): deprecate
V8_INLINE void Dispose() { Reset(); }
V8_DEPRECATED(V8_INLINE void Dispose(Isolate* isolate)) { Reset(); }
```