Use `priority: .default` rather than `.high` to replicate `.global()`
Use `priority: .default` rather than `.high` to replicate `.global()`

This is only relevant for code running on versions of OSX < 10.10.

`DispatchQueue.global()` is defined as:

```
public class func global(qos: DispatchQoS.QoSClass = default) -> DispatchQueue
```

Since the default value for qos is `.default`, I thought it made sense to
use `.default` for the priority as well.

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