See https://circleci.com/blog/sunsetting-1-0/:

TL;DR: After August 31, 2018, CircleCI 1.0 will no longer be available for Linux and macOS users.

We're currently using CircleCI 1.0 for this project (/cc @neelance):

Need to deal with it by the time it's shut down. Options include:

• migrate to CircleCI 2.0 (if it's still free)
• migrate to another CI