Update rake to 0.8.4.

Changes:
=== New Features / Enhancements in Version 0.8.4

* Case is preserved on rakefile names. (patch from James
  M. Lawrence/quix)

* Improved Rakefile case insensitivity testing (patch from Luis
  Lavena).

* Windows system dir search order is now: HOME, HOMEDRIVE + HOMEPATH,
  APPDATA, USERPROFILE (patch from Luis Lavena)

* MingGW is now recognized as a windows platform.  (patch from Luis
  Lavena)

=== Bug Fixes in Version 0.8.4

* Removed reference to manage_gem to fix the warning produced by the
  gem package task.

* Fixed stray ARGV option problem that was interfering with
  Test::Unit::Runner. (patch from Pivotal Labs)

=== Infrastructure Improvements in Version 0.8.4

* Numerous fixes to the windows test suite (patch from Luis Lavena).

* Improved Rakefile case insensitivity testing (patch from Luis
  Lavena).

* Better support for windows paths in the test task (patch from Simon
  Chiang/bahuvrihi)

(minskim)