Fix build breakage caused by xcode 7.3.1 upgrade compiler issue
Fix build breakage caused by xcode 7.3.1 upgrade compiler issue

proversity-org/edx-app-ios,appsembler/edx-app-ios,edx/edx-app-ios,appsembler/edx-app-ios,edx/edx-app-ios,Ben21hao/edx-app-ios-new,Ben21hao/edx-app-ios-new,Ben21hao/edx-app-ios-enterprise-new,edx/edx-app-ios,proversity-org/edx-app-ios,Ben21hao/edx-app-ios-enterprise-new,edx/edx-app-ios,Ben21hao/edx-app-ios-enterprise-new,Ben21hao/edx-app-ios-new,appsembler/edx-app-ios,proversity-org/edx-app-ios,Ben21hao/edx-app-ios-new,edx/edx-app-ios,appsembler/edx-app-ios,proversity-org/edx-app-ios,proversity-org/edx-app-ios,Ben21hao/edx-app-ios-enterprise-new,Ben21hao/edx-app-ios-new,appsembler/edx-app-ios,proversity-org/edx-app-ios,edx/edx-app-ios,edx/edx-app-ios,Ben21hao/edx-app-ios-enterprise-new