Remove format for journals 'created_at' field (#428)

- elasticsearch >= 2 will complain about the format, because
  `basic_date_time` is specified and epoch is given, while es < 2 will
  silently accept epoch
- removing the format should make es < 2 accept the epoch value as usual
  and let es >= 2 use its default
  `strict_date_optional_time||epoch_millis`

lib/chewy/journal.rb

@@ -7,7 +7,7 @@ class Journal
           type_name: { type: 'string', index: 'not_analyzed' },
           action: { type: 'string', index: 'not_analyzed' },
           object_ids: { type: 'string', index: 'not_analyzed' },
-          created_at: { type: 'date', format: 'basic_date_time' }
+          created_at: { type: 'date' }
         }
       }
     }.freeze