Update requirements to iOS 6 and OS X 10.8

It looks like the Xcode project specifies a minimum or OS X 10.8 SDK, and uses iOS 6.0 only features (strong properties with a type of `dispatch_queue_t`)
Minimum iOS version required for the Xcode project is 8.0
Merge pull request #1625 from tonyarnold/patch-1

Update requirements to iOS 8.0 and OS X 10.8