You can set the default ruby version you want to work on per project basis using rvm.
All you have do is to add .rvmrc file inside your project directory and add these lines to your file
Save the file and quit.
Now get out of the project directory and then cd into it.
This time it asks for permission to use the newly created rvmrc file.
Now your whole project will default to ruby version 1.9.3