Upgrade RuboCop to 0.25.0

* remove previously added temporary config hack from tests
* enable [`InlineComment`](https://github.com/bbatsov/rubocop/commit/5bc723b447fe2fe04691f3cd2da643a2a8d059a1) cop
Check for trailing commas

Use a trailing comma after each item in a multi-line list, including the last item.

> [Example](https://github.com/thoughtbot/guides/blob/master/style/samples/ruby.rb#L49)