Log Message:
Update ruby-plist to 3.2.0.

* Updated dprecated File.exists? to File.exist?
* Updated README.rdoc 'Apple Computer' -> 'Apple'
* Fixed defect in PData in which exception was thrown when <data/> element
  was read from plist.
* guard against not being able to load rdoc tasks
* Fix typo in comment
* Remove 'Computer' from 'Apple Computer' string in plist files ;-)
* Changed sort to sort_by in Plist::Emit.plist_node to allow mixed symbol and
  string hash keys.