Scenario outlines make the JSON formatter fail (an exception is raised)

The scenario added in this PR with the @wip-new-core tag, does pass in v1.3.x (but it is not included in the json_formatter.feature until #700).

The problem is that in gherkin_formatter_adapter the code relies on using the gherkin_statement method on the AST classes. This does not work on the AST classes Core::Ast::ScenarioOutline nor Core::Ast::ExamplesTable.

Requires cucumber/cucumber-ruby-core#59