Restore support for Ruby 2.3

[mvz]

Summary

Lowers the required ruby version to 2.3 and makes the necessary changes to the code to make that work.

Details

• Lower ruby versions in gemspec, rubocop configuration and Dockerfile
• Use most recent version of rubocop that still supports Ruby 2.3
• Undo several code changes that broke 2.3 compatibility

Motivation and Context

This will allow RSpec 4 to use Aruba 1 for its cucumber scenario's, rather than staying on super-ancient 0.14.x. See also https://github.com/rspec/rspec/issues/61#issuecomment-786505319 and #787.

How Has This Been Tested?

This was split off from #787 which was already tested in CI. Using the Dockerfile allows easy testing in a Ruby 2.3 environment.

Types of changes

• New feature (non-breaking change which adds functionality)
• Documentation update

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[mvz]

Current plans for RSpec are to keep the current cucumber + aruba dependencies and only upgrade after the 4.0 release. This means this PR can be on hold until then.

[olleolleolle]

Super to have the context for the plan, as well as the plan itself!

[mvz]

It is no longer realistic from a maintenance standpoint for Aruba 1.x or upcoming 2.x to support Ruby 2.3. RSpec will have to stay on 0.14.x until they drop support for 2.3.