[nahname]

You should install a ruby version manager. Most apps will specify the version in the project (.rvmrc or .ruby-version). If you have a ruby version manager and the required version installed, your system will switch automatically. Installing different versions of ruby is just a one liner.

  rvm instal ruby-1-9-3-p429

RVM setup guide: http://www.stewgleadow.com/blog/2011/12/10/installing-rvm-on...

rbenv setup guide: https://gist.github.com/jasoncodes/1223731