Group content in specialist navigation by its format

This pull request changes the specialist sector navigation to group the content returned by the format type, to make it easier to browse long lists of content.

The grouping of content actually happens in the Content API (see alphagov/govuk_content_api#160), and so this branch changes the request to the Content API to include `group_by=format` in the URL.

This also includes the first Cucumber tests in Frontend, and so I've set up cucumber-rails to run feature specs.