title: reinstall every gem for each ruby version?
body: <p>I just installed Ruby 2.0.0 using rbenv and set it to the global ruby version for my system. Since 2.0 is compatible with 1.9.3, I tried to start up a Rails project with it, but got the following error. I did <code>rbenv rehash</code> after installing 2.0</p>
  
  <pre><code>The `rails' command exists in these Ruby versions:
    1.9.3-p327
  </code></pre>
  
  <p>Does this mean that every gem I installed on my system with 1.9.3 has to be reinstalled if I wish to use it with 2.0?</p>