Move the linter gem to the test group
Move the linter gem to the test group

The new Jenkins2 jobs don't install gems that are only in the `development`
group.

We need to move the gem to be able to run the govuk ruby linter against our code
in Jenkins CI.

alphagov/smart-answers,aledelcueto/smart-answers,alphagov/smart-answers,alphagov/smart-answers,alphagov/smart-answers,aledelcueto/smart-answers,aledelcueto/smart-answers,aledelcueto/smart-answers