update Podfile to stick to latest release of AFNetworking 1.3 branch

AFNetworking 1.3.3 has a fix (https://github.com/AFNetworking/AFNetworking/commit/748260af04f15fedfbf894f2e56d8b37a8d6c237)  that solves this bug I ran into re: image orientation https://github.com/x2on/FSImageViewer/issues/3