Fix the ability spec

* Get rid of the volunteer coordinator role, we don't use it currently
* Test the ability with users that exclusively have a single role

app/controllers/admin/registrations_controller.rb

@@ -1,7 +1,7 @@
 module Admin
   class RegistrationsController < Admin::BaseController
     load_and_authorize_resource :conference, find_by: :short_title
-    load_and_authorize_resource through: :conference
+    load_and_authorize_resource :registration, through: :conference
     before_filter :set_user, except: [:index]
 
     def index

app/models/ability.rb

@@ -122,7 +122,7 @@ class Ability
     # can view others
     can :show, User
     # can register
-    can [:show, :create], Registration do |registration|
+    can [:read, :create], Registration do |registration|
         registration.new_record?
     end
   end