Bump rake dependency to 0.8.4. [#5279 state:resolved]

This rake version adds the Rake::RDocTask#rdoc_task_name method, used in
railties/lib/rails/tasks/documentation.rake
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

railties/railties.gemspec

@@ -19,7 +19,7 @@
   s.rdoc_options << '--exclude' << '.'
   s.has_rdoc = false
 
-  s.add_dependency('rake',          '>= 0.8.3')
+  s.add_dependency('rake',          '>= 0.8.4')
   s.add_dependency('thor',          '~> 0.14.0')
   s.add_dependency('activesupport', version)
   s.add_dependency('actionpack',    version)