Updated for arm64 compatibility

Note that Xcode requires iOS6 as the earliest deployment platform for cross-
compilation of arm32 / arm64 code.

Note that this will require some coordination with SocketRocket, and I have a pull request there for the same reason: https://github.com/square/SocketRocket/pull/120

We're making these fixes internally because we are developing iOS 7 apps and are trying to be native on arm64.

If you don't want to drop iOS5 compatibility, I understand.

Thanks for considering my pull request!