Add explicit != nil checks to fix archive/release build in Xcode 6 beta 5

Fixes three compiler errors:
* Alamofire.swift:120:20: Optional type 'NSData!' can not be used as a boolean; test for '!= nil' instead
* Alamofire.swift:128:20: Optional type 'NSData!' can not be used as a boolean; test for '!= nil' instead
* Alamofire.swift:681:16: Optional type '((Int64, Int64, Int64) -> Void)!' can not be used as a boolean; test for '!= nil' instead