(specify your favorite flavor if you're not ready for 1.9.3):
$ rbenv install 1.9.3-p0
to rebuild rbenv's shims, when you install a new system system gem with a binary (like rake):
$ rbenv rehash
set a global version of Ruby to be used (instead of your system Ruby):
$ rbenv global 1.9.3-p0
set the version of Ruby for a specific project directory by creating a .rbenv-version file (which we ignored earlier):
$ rbenv local 1.9.2-p290
to get bundler installed into your current Ruby's gemset:
$ gem install bundler
Run bundle in your project directory will now install the gems in your Gemfile directly into your project in vender/bundle.
Finally, to start up your app in the context of your bundled gems:
$ bundle exec rails s