Upgrade to Chef 14.9

Chef 14.9 appears to support Ruby 2.6. Most of the changes related to Ruby 2.6 updates appear to be a simple rename of Net::HTTPServerException to Net::HTTPClientException (https://github.com/chef/chef/pull/7922/files ), so I don't think it necessarily blocks an upgrade to Ruby 2.6 if we can live with deprecation warnings.

For reference, this was the merge request that upgrade to Chef 13.6.4: !2279 (merged)