Please update iOS building procedure to cmake 3.14

[crazyhappygame]

Starting from cmake 3.14.0 there is native support for iOS:
https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html#cross-compiling-for-ios-tvos-or-watchos

Building with CMake for iOS
https://github.com/firebase/firebase-cpp-sdk/blob/master/README.md#building-with-cmake-for-ios

Should not require -DCMAKE_TOOLCHAIN_FILE=../cmake/ios.cmake with CMake 3.14 and above

[stewartmiles]

@crazyhappygame the min cmake version we support is 3.1, we're happy to accept pull requests if this can be applied conditionally.