title: How to change ruby version
body: <p>Ruby 1.8 and 1.9 are installed in my Ubuntu machine. I've just installed Ruby 2.0.0 from <code>ppa:brightbox/ruby-ng-experimental</code> to install a bundle which requires 2.0.0. Now all 1.8, 1.9 and 2.0 are installed though I can't tell bundle to use 2.0:</p>
  
  <pre><code>$ bundle install
  $ Your Ruby version is 1.9.3, but your Gemfile specified 2.0.0
  </code></pre>
  
  <p>RVM fails to change version:</p>
  
  <pre><code>$ rvm 2.0
  $ ruby-2.0.0-p451 is not installed.
  $ To install do: 'rvm install ruby-2.0.0-p451'
  </code></pre>
  
  <p>RBENV also does not recognize 2.0:</p>
  
  <pre><code>$ rbenv global 2.0.0-p451
  $ rbenv: version `2.0.0-p451' not installed
  </code></pre>