The test bundler group currently contains spork, which is a pretty inactive project that also does not support Rails 5, so I'd like to replace it.

Rails 4.1 introduced spring by default in new applications to preload Rails for both test and development environments, so it works to replace use of spork for tests (bin/rake test test/unit/domain_test.rb) and also speeds up console/runner usage etc. https://github.com/rails/spring#readme has more usage info.