title: Changing Ruby versions with rbenv from system and "shell" command does not work
body: <p>I can change my rbenv superficially with local and global commands but my Ruby version does not fundamentally change.  My shell command is not recognized but local and global are.  I had RVM previously but uninstalled (as far as I know).  What should I do?</p>
  
  <pre><code>$ rbenv versions
    system
  * 1.9.3-p0 (set by /Users/geoffreylord/.rbenv/version)
    2.0.0-p247
  $ rbenv local
  rbenv: no local version configured for this directory
  $ rbenv global
  1.9.3-p0
  $ rbenv shell
  rbenv: no such command `shell'
  $ ruby -v
  ruby 1.8.7 (2012-02-08 patchlevel 358) [universal-darwin12.0]
  </code></pre>