Ensure no leftover RuboCop version checks

[sambostock]

As we may increase the minimum RuboCop version requirement soon, we should make sure that if we do, we don't leave behind redundant version checks.

This adds a test to enforce they are cleaned up.

Example failure caused by adding a line checking against an old RuboCop version:

  1) Failure:
ConfigTest#test_config_does_not_check_for_rubocop_versions_below_minimum_version [test/config_test.rb:113]:
Error: The following RuboCop version checks are redundant given the required version in the gemspec:

     213: <% if rubocop_version >= "1.1" %>

Please remove these checks, or (if you intend to maintain compatibility) loosen the `rubocop` requirement in the gemspec.