Fixes Xcode 8 beta 4 compatibility with its Swift 3 / SDK version: no .serial attribute anymore in DispatchAttributes (presumably as it's described by absence of attributes).
Fixes Xcode 8 beta 4 compatibility with its Swift 3 / SDK version: no .serial attribute anymore in DispatchAttributes (presumably as it's described by absence of attributes).

emaloney/CleanroomASL,emaloney/CleanroomASL