Add frozen_string_literal magic comment

re: https://www.rubydoc.info/gems/rubocop/RuboCop/Cop/Style/FrozenStringLiteralComment

app/serializers/conference_serializer.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class ConferenceSerializer < ActiveModel::Serializer
   include ApplicationHelper
   attributes :short_title, :title, :description, :start_date, :end_date, :picture_url,

app/serializers/conferences_array_serializer.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 #
 # Needed in order to add the API version number to the conferences array
 #

app/serializers/event_schedule_serializer.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class EventScheduleSerializer < ActiveModel::Serializer
   include ActionView::Helpers::TextHelper
 

app/serializers/event_serializer.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class EventSerializer < ActiveModel::Serializer
   include ActionView::Helpers::TextHelper
 

app/serializers/room_serializer.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class RoomSerializer < ActiveModel::Serializer
   attributes :guid, :name, :description
 

app/serializers/speaker_serializer.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class SpeakerSerializer < ActiveModel::Serializer
   include ActionView::Helpers::TextHelper
 

app/serializers/track_serializer.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class TrackSerializer < ActiveModel::Serializer
   attributes :guid, :name, :color
 end