Log Message:
Update ruby-activerecord to 1.15.5.

Changes:
* Depend on Action Pack 1.4.4
* Fix #count on a has_many :through association so that it recognizes the
  :uniq option. Closes #8801 [lifofifo]
* Don't clobber includes passed to has_many.count [danger]
* Make sure has_many uses :include when counting [danger]
* Save associated records only if the association is already loaded.  #8713
* Changing the :default Date format doesn't break date quoting.  #6312
* Allow nil serialized attributes with a set class constraint. #7293
* belongs_to assignment creates a new proxy rather than modifying its target
  in-place.  #8412 [mmangino@elevatedrails.com]
* Fix column type detection while loading fixtures.  Closes #7987 [roderickvd]
* Document deep eager includes.  #6267 [Josh Susser, Dan Manges]
* Oracle: extract column length for CHAR also.  #7866 [ymendel]
* Small additions and fixes for ActiveRecord documentation.  Closes #7342
* SQLite: binary escaping works with $KCODE='u'.  #7862 [tsuka]
* Improved cloning performance by relying less on exception raising #8159