KATTS-2621 Change method stub syntax, change 'should' syntax to 'expect'...

..., change scope for method stub to be 'each' instead of 'all', and rename 'negative_failure_message' method to 'failure_message_when_negated' method in custom Matchers. All changes stem from deprecation warnings and test failures.

Also update README.md to reflect Ruby version this was locally tested against.