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). 
Very neat -- I've added this, but dropped the `Processing.instances = []` line, since `exit()` slices the instance out of the array (in fact, it'd be a bad idea to reassign `Processing.instances` anyway since it needs to remain an alias of `processingInstances`).