Add CHANGELOG entry

actionpack/CHANGELOG.md

 ## Rails 3.2.0 (unreleased) ##
 
+*   Depends on rack ~> 1.4.0 *Santiago Pastorino*
+
 *   Add :gzip option to `caches_page`. The default option can be configured globally using `page_cache_compression` *Andrey Sitnik*
 
 *   The ShowExceptions middleware now accepts a exceptions application that is responsible to render an exception when the application fails. The application is invoked with a copy of the exception in `env["action_dispatch.exception"]` and with the PATH_INFO rewritten to the status code. *José Valim*