Installing Rails 3.1.x

$ mkdir rails31
$ cd rails31
$ bundle init
Writing new Gemfile to /infra/lamp/mounts/emc256/emc256test2/rails/rails31/Gemfile
$ editor Gemfile
$ cat Gemfile
source "http://rubygems.org"

gem "rails","3.1.0"
gem "mysql2"
$ bundle install --path vendor
Fetching source index for http://rubygems.org/
Installing rake (0.9.2) 
Installing multi_json (1.0.3) 
Installing activesupport (3.1.0) 
...
$ bundle exec rails -v
Rails 3.1.0
$ bundle exec rails new ../rails31 -d mysql < /bin/echo "Y"
...
$ cat Gemfile
source 'http://rubygems.org'

gem 'rails', '3.1.0'

# Bundle edge Rails instead:
# gem 'rails',     :git => 'git://github.com/rails/rails.git'

gem 'mysql2'
...