postNotificationTest: Remove deprecated GCD func
postNotificationTest: Remove deprecated GCD func

`dispatch_get_current_queue()` is deprecated as of OS X 10.9. Luckily,
we know XCTest test cases are always run serially on the main queue, so
we can replace this call with a simple `dispatch_get_main_queue()`
instead. This fixes a compiler warning.

specta/expecta,jmoody/expecta,PatrykKaczmarek/expecta,iosdev-republicofapps/expecta,Lightricks/expecta,jmoody/expecta,iguchunhui/expecta,PatrykKaczmarek/expecta,PatrykKaczmarek/expecta,iosdev-republicofapps/expecta,udemy/expecta,PatrykKaczmarek/expecta,iosdev-republicofapps/expecta,suxinde2009/expecta,jmoody/expecta,suxinde2009/expecta,wessmith/expecta,Bogon/expecta,Bogon/expecta,iguchunhui/expecta,jmoody/expecta,Lightricks/expecta,wessmith/expecta,udemy/expecta,iosdev-republicofapps/expecta