Depend only on cucumber-core at runtime

[mvz]

Summary

Limit Aruba's runtime dependency on cucumber to just cucumber-core.

Details

This limits Aruba's runtime dependencies on the cucumber ecosystem to just cucumber-core. Aruba uses cucumber-core for its event bus implementation

Motivation and Context

This makes using Aruba with RSpec or minitest less awkward by not pulling in the entirety of Cucumber. See #402 and #806 for background on this.

How Has This Been Tested?

CI.

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]

This requires some update to the documentation to make sure users pull in full cucumber if they need it.