Remove iOS arm64 target and validate project settings for Xcode 5.1

The `ObjectiveGit-iOS-arm64` target was created with the intent of preserving backwards compatibility with Xcode 4.6. (Maintaining Travis support.) Almost 6 months later, and another major release of Xcode behind us, the `ObjectiveGit-iOS-arm64` target has outlived it's purpose. (ObjectiveGit doesn't even use Travis anymore.) 

This PR removes the target in favor of supporting arm64 in the `ObjectiveGit-iOS` target. In addition this also validates the project settings for Xcode 5.1. (But not submodules) 

// @alanjrogers