From February 1st 2015 all new iOS apps submitted to the App Store must include 64-bit support and be built with the iOS 8 SDK.
Beginning June 1st 2015 app updates will also need to follow the same requirements.
To enable 64-bit in your project Apple recommend using the default Xcode build setting of “Standard architectures” to build a single binary with both 32-bit and 64-bit code.