Fix bug in reload/re-init behaviour

Processing instances were not being unloaded properly when Processing.reload() was called, since the `processingInstances` variable is no longer in scope at that point (since the post-v1.4.1 refactor). 
That's true, totally forgot about that. But, since once it's empty any reversals become moot, we can just add a reverse() call:
```
Processing.instances.reverse().forEach(function(sketch) {
  sketch.exit();
});
```