1. Create a Gemfile
source "https://rubygems.org"
ruby '2.2.3'
gem 'rack', github: 'rack/rack'
gem 'rails', git: 'git://github.com/rails/rails.git'
gem 'arel', git: 'git://github.com/rails/arel.git'
2. bundle
3. bundle exec rails new . --dev --force
4. bundle exec rails s
Go to localhost:3000, you should now see 5.0.0.alpha in the environment.
Reference
Setting up Rails 5 app from edge
