Add Rubocop exemption for the Style/ClassAndModuleChildren rule, so t…

[StephenWetzel]

…hat the openssl_fix.rb file passes.

As referenced here, this should fix CI which broke in https://github.com/njh/ruby-mqtt/pull/139.

Let me know if you'd prefer to keep this rule and I will update the formatting of openssl_fix.rb to comply with the rule, although @lucaong reasoning in that issue convinced me to just disable the rule.

Then ideally you can also merge https://github.com/njh/ruby-mqtt/pull/133 and cut a new release.