title: how to solve "ruby installation is missing psych" error?
body: <p>I used rvm to install ruby 1.9.3. even though it was successfully installed, it complained about libyaml. and now every time i wanna install a gem (say rails) this warning shows up:</p>
  
  <pre><code>It seems your ruby installation is missing psych (for YAML output). To eliminate this warning, please install libyaml and reinstall your ruby.
  </code></pre>
  
  <p>I use Mac os X 10.7 (Lion).</p>