fixed crash on arm64 devices (issue : https://github.com/square/PonyDebugger/issues/84)

Due to Apple`s documentation about arm 64 bit ," function pointers must use the correct prototype".
If not , all parameters will be nil, which results in a crash of AFNetworking.

(https://developer.apple.com/library/ios/documentation/General/Conceptual/CocoaTouch64BitGuide/ConvertingYourAppto64-Bit/ConvertingYourAppto64-Bit.html#//apple_ref/doc/uid/TP40013501-CH3-SW22)