Multithreading Violation while saving parent context

[saurabytes]

Overview

I have enabled concurrency detection for multithread violations using
-com.apple.CoreData.ConcurrencyDebug 1 in the Run arguments
Since then every save I raising this flag. Currently it seems this is happening how the saves are being handled.
The violation happens in save context for parent after the NSManagedObjectContextDidSaveNotification is posted for the child context

#### Steps 1. Update `Run` in scheme for ExampleApp to have [`-com.apple.CoreData.ConcurrencyDebug 1`](http://oleb.net/media/xcode-scheme-core-data-concurrency-debug.png) 2. Add a company in the app 3. The app crashes with above stack trace `+[NSManagedObjectContext__Multithreading_Violation_AllThatIsLeftToUsIsHonor__] ()`

[saurabytes]

Fix

Looking more closely and verify with a test this is the resolution
The context hasChanges verification has to be thread safe too

    let block = {
        guard context.hasChanges else { return }
        do {
            try context.save()
            completion?(.Success)
        }
        catch {
            completion?(.Failure(error as NSError))
        }
    }

[jessesquires]

Thanks @saurabytes ! 👍

Closing as a duplicate of #59.

If you would like to submit a PR, please do! 😄