Add doesntExist to passthru

Those calls should return the same result but instead the call to "doesntExist" returns an object.

```php
!$user->groups()->exists();
$user->groups()->doesntExist();
```

Author: simonschaufi

src/Jenssegers/Mongodb/Eloquent/Builder.php

@@ -26,6 +26,7 @@ class Builder extends EloquentBuilder
         'avg',
         'sum',
         'exists',
+        'doesntExist',
         'push',
         'pull',
     ];