Vendoring rails using a symlink
Been meaning to do this for ages!
First of all clone rails from git hub into a suitable folder
git clone git://github.com/rails/rails.git rails.git
I like to give repositories like this a .git extension so can easily differentiate from local things, but you can call the folder whatever you want.
After the repository is cloned be sure to checkout the branch you want, in my case
git checkout origin/2-3-stable
Now you can vendor your rails projects by going into the root folder of the project and running
ln -s /path_to/rails.git vendor/rails
Adjust the path as required.