Subclass `Exception` rather than `StandardError`.
Upgrade cucumber to latest.
Improve integration with minitest.

- Support minitest 5.x rather than 4.x.
- Make expectations count as assertions in minitest's runner.
- Make expectation failures report as failures, not errors.
- Stop auto-integrating; users can require a file instead.
  I think this fits better with minitest's philosophy, and
  there are situations (such as in a rails app) where minitest
  is loaded, but the runner is not being used.
- Stop integrating with test/unit. T::U doesn't appear to be
  used much these days. Users can still integrate it themselves
  (by manually mixing in `RSpec::Matchers`). Stdlib T::U is not
  usable while minitest 5.x is installed, so there's not an easy
  way to run builds against T::U while we also run builds against
  minitest 5.x.