Add Eventserializer tests and fix undefined method `l' for Eventserializer

Solution of no method error pointed by @nishanthvijayan:
http://stackoverflow.com/a/11724935

app/serializers/event_serializer.rb

@@ -5,7 +5,7 @@ class EventSerializer < ActiveModel::Serializer
 
   def date
     t = object.start_time
-    t.blank? ? '' : %{ #{l t, format: :short}#{t.formatted_offset(false)} }
+    t.blank? ? '' : %{ #{I18n.l t, format: :short}#{t.formatted_offset(false)} }
   end
 
   def speaker_ids