Xcode 9.1 beta is now available! 
To select this Stack you just have to open your app on bitrise.io,
go to the Settings tab, and at the bottom of the Settings tab select the Xcode 9.1.x
stack option, and your next build will start on the Xcode 9.1 stack.
You can find the Stack’s full system report at:
https://github.com/bitrise-io/bitrise.io/blob/master/system_reports/osx-xcode-9.1.x.log.
If you’d like to add additional tools to be pre-installed you can find the instructions on GitHub, for both the Linux and for the macOS stacks.
Information about Stack types & update schedules can be found here: discuss.bitrise.io/t/281.
Happy Building! 