KWSpec: Cast objc_msgSend for 64-bit device.

Fixes issue #479. See Apple's 64-bit Transition Guide for details:

https://developer.apple.com/library/ios/documentation/General/Conceptual/CocoaTouch64BitGuide/ConvertingYourAppto64-Bit/ConvertingYourAppto64-Bit.html
Merge pull request #480 from modocache/fix-479-64-bit-cast-msgsend

KWSpec: Cast objc_msgSend for 64-bit device.