Mac OSX Ruby on Rails Installation problem - file not found / error with generating documentation

I am installing Ruby on Rails on OSX. I am following these instructions:

tp> tar xzf rubygems-x.y.z.tar.gz
tp> cd rubygems-x.y.z
rubygems-x.y.z> sudo ruby setup.rb

All good up to here and then I issue with this command with the following error:

tp> sudo gem install rails --include-dependencies
INFO:  `gem install -y` is now default and will be removed
INFO:  use --ignore-dependencies to install only the gems you list
Successfully installed rails-3.0.5
1 gem installed
Installing ri documentation for rails-3.0.5...
File not found: lib
ERROR:  While ge开发者_StackOverflow中文版nerating documentation for rails-3.0.5
... MESSAGE:   exit
... RDOC args: --ri --op /Library/Ruby/Gems/1.8/doc/rails-3.0.5/ri lib --title rails-3.0.5 Documentation --quiet

Additional info:

My version of Ruby:

sh-3.2# ruby -v
ruby 1.8.7 (2009-06-12 patchlevel 174) [universal-darwin10.0]

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I recommend you to use RVM for working with Ruby and Ruby Gems:

http://rvm.io