ci: Upgrade Travis builds from Xcode 9.3 -> Xcode 10.1
ci: Upgrade Travis builds from Xcode 9.3 -> Xcode 10.1

This will allow the pre-built frameworks attached to releases to get built with the latest Swift
version (4.2.1)

watson-developer-cloud/ios-sdk,watson-developer-cloud/ios-sdk,watson-developer-cloud/ios-sdk,watson-developer-cloud/ios-sdk,watson-developer-cloud/ios-sdk