Fix support for old iOS sdks (namely iOS 7.1 SDK)

We still have until February 1st, 2015 to submit apps with iOS 7.1 [1]. So whilst it's trivial to support we might as well continue to support the iOS 7.1 SDK.

[1] http://9to5mac.com/2014/10/20/apple-to-require-ios-apps-and-updates-to-use-ios-8-sdk-and-include-64-bit-support-from-february-2015/