Xcode 9.3.1 is now available! 
To select this Stack you just have to open your app on bitrise.io,
go to the Workflow tab (Workflow Editor), and on the Stack tab select the Xcode 9.3.x
stack option, and your next build will start on the Xcode 9.3.1 stack. Note: you can now select separate stacks for separate workflows! Read more at: https://blog.bitrise.io/per-workflow-stack-selection-is-here.
This new Virtual Machine replaced the previous Xcode 9.3 configuration,
so if you’ve already selected the Xcode 9.3.x stack for your app, you’re automatically using Xcode 9.3.1 now.
You can find the Stack’s system report at:
https://github.com/bitrise-io/bitrise.io/blob/master/system_reports/osx-xcode-9.3.x.log,
and the changes/updates compared to the Xcode 9.3 stack at:
https://github.com/bitrise-io/bitrise.io/pull/213/files.
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! 