We have released the Xcode 9.1 image on CircleCI 2.0. Please add the following to your .circleci/config.yml to select that version of Xcode in your jobs:
macos:
xcode: "9.1.0"
You can find more information on the content of the image in the Xcode 9.1 image announcement.
If you are currently not using CircleCI 2.0 for iOS but would like to try, please submit a beta access request here.