Fix crash when OS_OBJECT_USE_OBJC=0 is set on build

I noticed my app always crash when build for iOS 6 with rubymotion.

At commit ec8b8ef OS_OBJECT_USE_OBJC check is added to EvernoteSession. The
declaration of queue should also check this flag, such that when
OS_OBJECT_USE_OBJC=1, the compiler treat queue as object (and thus retain it properly)