I had issues deploying this on a heroku-18 stack with the version of ruby defined. Segfaults were occurring from a 4.0.3 activesupport dependency, supplied by the ruby package system when using the defined ruby versions (2.2? 2.3?).
This pull request upgrades everything to 2.5.1 ruby, including all of its dependencies.
Tests pass after rebuilding the dockerimage (updated to extend from RUBY:2.5)
Let me know if there's a better way of defining these version restrictions. It's possible it might work with 2.4 well, but I haven't had the time to try. The ruby buildpack does not support Ruby 2.3 running on the latest stack (heroku-18).