I’m running into the same issue. I’ve tried using the newer Ubuntu image (14.04) - test pass, but I need the heroku toolbelt to deploy (and that’s not available on the Ubuntu 14.04 environment).
The issue seems to be with the Ruby 2.3.1 upgrade; downgrading solved it for me (but that’s not really an option).
Rebuild without cache works, but I have to do it every time I push a branch with a new name. This seems like a bug. It could be fixed with a clear cache button but I can’t find one?
it’s weird, I got that issue just when my branch is already in circleci then update ruby version for that branch but when I push a new branch with new ruby version everything work well.