Force cucumber-message version to 6.x

[vincent-psarga]

Summary

Until we can support cucumber-messages 7, we'll force the use of the 6.0 releases.

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

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)

Checklist:

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

[olleolleolle]

This helps users avoid getting stuck with versions not supported.

[aslakhellesoy]

The reason for this change is to be able to make releases of dependent libraries (cucumber-messages in this case) without breaking libraries that depend on them (cucumber-ruby-core in this case).

We'll update cucumber-ruby-core to use the latest version of cucumber-messages in due course.