Error installing rails bundler requires RubyGems version >= 1.3.6

sudo gem install railsでこんなエラーが出ちゃいました。

Error installing rails bundler requires RubyGems version >= 1.3.6

解決策は

sudo gem update --system

pdating RubyGems
Updating rubygems-update
Successfully installed rubygems-update-1.6.1
Updating RubyGems to 1.6.1
Installing RubyGems 1.6.1
RubyGems 1.6.1 installed

=== 1.6.1 / 2011-03-03

Bug Fixes:

#  Installation no longer fails when a dependency from a version that won't be
  installed is unsatisfied.
#  README.rdoc now shows how to file tickets and get help.  Pull Request #40 by
  Aaron Patterson.
#  Gem files are cached correctly again.  Patch #29051 by Mamoru Tasaka.
#  Tests now pass with non-022 umask.  Patch #29050 by Mamoru Tasaka.


------------------------------------------------------------------------------

RubyGems installed the following executables:
    /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/gem

を実行した後にsudo gem install railsでrailsをインストールすればOKです。