Use modern rspec syntax

>Deprecation Warnings:
>
>Using `stub` from rspec-mocks' old `:should` syntax without explicitly enabling the syntax is deprecated. Use the new `:expect` syntax or explicitly enable `:should` instead. Called from /.../danger-code_style_validation/spec/code_style_validation_spec.rb:18:in `block (3 levels) in <module:Danger>'.