You missed a few steps:

  brew install rbenv
  brew install ruby-build

Copy the line it gives you to your shell's rc file and source it/open a new term:

  rbenv install 1.9.3-p125
  rbenv global 1.9.3-p125
  gem update --system
  gem install rails
  gem install bundler # optional, recommended

When a new version of Ruby comes out:

  brew update
  brew upgrade

Then just rbenv install and rbenv global it.