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). 
Not a bad idea, but then I'd suggest calling exit on all entries first, and then wiping:
```
Processing.instances.forEach(function(sketch) {
  sketch.exit();
});
Processing.instances = [];
```