CHEF-2541 Restore rubygem reference

[vkarve-chef]

Description

Restoring rubygems reference from chefspec-chef to chefspec

Related Issue

As we have push access to this gem in rubygems, we can allow users to continue with the same gem name.

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 change)
• Chore (non-breaking change that does not add functionality or fix an issue)

Checklist:

• I have read the CONTRIBUTING document.
• I have run the pre-merge tests locally and they pass.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commits have been signed-off for the Developer Certificate of Origin.