respond_to?(:chef_version) for < 12.6 compat

[zrisher]

Description

This PR checks if chef_version is defined before using it in metadata.rb.

Issues Resolved

This allows Chef clients < 12.6 to use this cookbook. See:

• chef_version setting in cookbook metadata  berkshelf/berkshelf#1499
• https://docs.chef.io/release_notes.html#what-s-new-in-12-6

Check List

• All tests pass. See https://github.com/chef-cookbooks/community_cookbook_documentation/blob/master/TESTING.MD
• New functionality includes testing.
• New functionality has been documented in the README if applicable
• All commits have been signed for the Developer Certificate of Origin. See https://github.com/chef-cookbooks/community_cookbook_documentation/blob/master/CONTRIBUTING.MD

[zrisher]

It appears that the failed tests do not relate to this change.

[tas50]

Thanks

[zrisher]

@tas50 Thank you!