Add frozen_string_literal comment

This fixes some warnings that Danger shows.

ee/spec/factories/project_aliases.rb

+# frozen_string_literal: true
+
 FactoryBot.define do
   factory :project_alias do
     project

ee/spec/models/project_alias_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe ProjectAlias do

ee/spec/requests/api/project_aliases_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe API::ProjectAliases, api: true do