Fix DashboardHelper reference in spec

Helpers::DashboardHelper doesn't actually exist, and Ruby 2.5 gets rid
of the automatic top-level constant lookup
(https://bugs.ruby-lang.org/issues/11547).

spec/features/user_sorts_things_spec.rb

@@ -6,7 +6,7 @@ require "spec_helper"
 # All those specs are moved out to this spec intentionally to keep them all in one place.
 describe "User sorts things" do
   include Spec::Support::Helpers::Features::SortingHelpers
-  include Helpers::DashboardHelper
+  include DashboardHelper
 
   set(:project) { create(:project_empty_repo, :public) }
   set(:current_user) { create(:user) } # Using `current_user` instead of just `user` because of the hardoced call in `assigned_mrs_dashboard_path` which is used below.