Support for upcoming RSpec 4.0

[mvz]

Summary

Several changes to allow use of Aruba with RSpec 4, and use of Aruba 1.0 to test RSpec 4.

Details

• Loosen dependency on rspec-expectations so Aruba users can upgrade to RSpec 4 when it comes out.
• Add back support for Ruby 2.3 to support RSpec's (tentative) wish to support that Ruby version for the 4.x series.

I may split this up later once it becomes clearer whether testing RSpec with cucumber 2, 3 or 4 is possible (currently each of these versions has some problem).

Motivation and Context

See https://github.com/rspec/rspec/issues/61#issuecomment-786505319 and subsequent comments for some discussion.

How Has This Been Tested?

CI for now. Hopefully the new 2.3 build targets will behave.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Internal change (refactoring, test improvements, developer experience or update of dependencies)

Checklist:

• I've added tests for my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[mvz]

Supporting Ruby 2.3 is no longer realistic. I will open a new pull request for Aruba to work with RSpec 4 when it comes out.