Add lint to the default rake task

Because this app uses minitest which runs via an at_exit hook this causes
the lint to run first, then the cucumber features, then, finally, the
tests.  This is probably the wrong way around in terms of speed and things
you care, but that's minitest for you.

Rakefile

@@ -6,5 +6,6 @@ if Rails.env.development? || Rails.env.test?
   require 'ci/reporter/rake/minitest'
 end
 
+task default: [:lint]
 Rails.application.load_tasks
 task test: :check_for_bad_time_handling

lib/tasks/lint.rake

@@ -0,0 +1,4 @@
+desc "Run govuk-lint with similar params to CI"
+task "lint" do
+  sh "bundle exec govuk-lint-ruby --rails --diff --format clang app test lib config"
+end