Test always against the latest Chef (point) releases on Travis

This PR allows CHEF_VERSION to include requirement specification, i.e. `"~> 10.16.4"` while keeping the backwards compatibility to use the exact version if no operator is used.

It also adds the current latest Chef version to the build matrix. Or actually the one that would be used by default gemspec. Currently v10.18.2.

And save some workload from Travis by building Ruby 2.0 only against the latest Chef version.