RuboCop URL in README uses http://rubocop.org

[dafyddcrosby]

Documentation still references http://rubocop.org for URLs, which works but isn't as secure as using https://rubocop.org (good newbie task)
See

cookstyle/README.md

Line 13 in 2971520

Cookstyle is powered by the [RuboCop](http://www.rubocop.org) linting engine. RuboCop ships with over three-hundred rules, or cops, designed to detect common Ruby coding mistakes and enforce a common coding style. We've customized Cookstyle with a subset of those cops that we believe are perfectly tailored for cookbook development. We also ship **254 Chef Infra specific cops** that catch common cookbook coding mistakes, cleanup portions of code that are no longer necessary, and detect deprecations that prevent cookbooks from running on the latest releases of Chef Infra Client.