Test with multiple versions of Cucumber

[mvz]

Summary

Add some mechanism to test that Aruba still works with older versions of Cucumber that are officially supported.

Expected Behavior

At least the combination of Aruba with Cucumber 2.4 is tested on TravisCI.

Current Behavior

Tests are run with the latest Cucumber version supported on each Ruby version. This means versions older than Cucumber 4 are not tested.

Possible Solution

Use the appraisal gem to test with different combinations of dependencies.

Context & Motivation

I'd like to start using more modern ways of defining the steps, and also improve step wording, but I'm afraid I'll break support for older Cucumber versions.

[mvz]

The upcoming end of support for Ruby 2.5 may be a good time to increase the minimum versions of Aruba's dependencies. This should mitigate this problem as well.

[mvz]

Adding Appraisals may also help in testing with upcoming Cucumber releases. See #801.

[aurelien-reeves]

Short summary of the discussion we had in #801:

We keep dependency to cucumber bounded to the actual version of cucumber.

That is source of issues when working on releasing new versions of cucumber. We had a really hard time releasing cucumber version 6 because of circular dependencies: cucumber depends on cucumber-wire which depends on aruba which depends on cucumber.

It has been suggested to remove the upper-bound dependency version. That way, releasing new version of cucumber would be easier.

It has also been suggested to have some tests for aruba using cucumber's master branch.

This is still an open discussion.