Enable Style/CommentAnnotation Rubocop cop

This cop checks that comment annotation keywords are written according
to guidelines. The offense was automatically corrected.

app/models/event_user.rb

@@ -1,5 +1,5 @@
 class EventUser < ActiveRecord::Base
-  # TODO Do we need these roles?
+  # TODO: Do we need these roles?
   ROLES = [%w[Speaker speaker], %w[Submitter submitter], %w[Moderator moderator]]
 
   belongs_to :event