Log Message:
Update ruby-activerecord to 2.3.8.  Patch by Wen Heping in PR 43441.

Changes:
* Add index length support for MySQL. #1852 [Emili Parreno, Pratik Naik]
* find_or_create_by_attr(value, ...) works when attr is protected.
  #4457 [Santiago Pastorino, Marc-Andr\xc3\xa9 Lafortune]
* JSON supports a custom root option: to_json(:root => 'custom') #4515
  [Jatinder Singh]
* Destroy uses optimistic locking. If lock_version on the record
  you're destroying doesn't match lock_version in the database, a
  StaleObjectError is raised.  #1966 [Curtis Hawthorne]
* To prefix the table names of all models in a module, define
  self.table_name_pr efix on the module.  #4032 [Andrew White]
* Association inverses for belongs_to, has_one, and
  has_many. Optimization to re duce database queries.  #3533 [Murray
  Steele]
* MySQL: add_ and change_column support positioning.  #3286 [Ben Marini]
* Reset your Active Record counter caches with the reset_counter_cache
  class method.  #1211 [Mike Breen, Gabe da Silveira]