travis ci: update config
travis ci: update config

alexeyxo/protobuf-objc,E-B-Smith/protobuf-objc,E-B-Smith/protobuf-objc,E-B-Smith/protobuf-objc,alexeyxo/protobuf-objc,alexeyxo/protobuf-objc,alexeyxo/protobuf-objc,E-B-Smith/protobuf-objc