`dispatch_release` excluded on 6.0+ (instead of 7.0+)

Changed `#if` wrapping around `dispatch_release` to check for iOS 6.0 or newer, rather than 7.0 or newer.

This fixes errors:
* `error: 'release' is unavailable: not available in automatic reference counting mode`
* `ARC forbids explicit message send of 'release'`

occurring on the `dispatch_release` call when compiling against iOS 6.x (checked on iOS 6.1; assumed the problem is 6.0+-relevant based on https://github.com/ReactiveCocoa/ReactiveCocoa/issues/204 and https://github.com/Nyx0uf/NYXImagesKit/issues/16).

Resolves BradLarson/GPUImage#1528