Add support for SimpleCov > 0.18.x

[etagwerker]

Right now skunk is not compatible with the latest release of SimpleCov -- more here: simplecov-ruby/simplecov#887

We should follow this path:

• Make the current version of skunk only compatible with SimpleCov < 0.18: https://github.com/fastruby/skunk/blob/master/skunk.gemspec#L52

• Wait for a JSON formatter that we can use moving forward. We will need this to support SimpleCov 0.18.x and higher: Support for relative paths in coverage/.resultset.json  simplecov-ruby/simplecov#887 (comment)

• Skunk should fail if we are trying to generate a report using a coverage/.resultset.json generated with v0.18.x -- The file structure is probably different between 0.17.x and 0.18.x

You will get unexpected results if you're trying to use a coverage/.resultset.json file generated with SimpleCov v0.18.x or higher.

[etagwerker]

@bronzdoc Update: The latest release of RubyCritic is compatible with the most recent versions of SimpleCov. So this should be simple: We can relax this dependency: https://github.com/fastruby/skunk/blob/main/skunk.gemspec#L52