title: How do I "activate" a different version of a particular gem?
body: <p>I want to switch between rails 2.3.10 as the "active" gem for my OS, so that I can invoke it at the command line.</p>
  
  <p>Is it possible to do this? I'm not using rvm. Maybe it's time to start.</p>
  
  <p>I tried <code>gem install rails --version=2.3.10</code>, but that just makes sure that version of the gem is installed, it doesn't put it in <code>/usr/bin/rails</code>.</p>
  
  <p>(I do already use bundler for my apps -- but haven't needed any precise control over gems at the OS level until now)</p>