Deprecation warnings for Swift 4

[phatmann]

/Users/tony/Source/next/next-ios/Pods/InputValidator/Sources/CardExpirationDateInputValidator.swift:37:56: 'substring(from:)' is deprecated: Please use String slicing subscript with a 'partial range from' operator.

/Users/tony/Source/next/next-ios/Pods/InputValidator/Sources/CardExpirationDateInputValidator.swift:53:57: 'substring(from:)' is deprecated: Please use String slicing subscript with a 'partial range from' operator.