title: Error when compiling PDF using knitr
body: <p>When I try to compile a PDF using knitr I get the following error:</p>
  
  <pre><code>there is no package called 'knitr' Error: object 'opts_knit' not found
  Execution halted
  </code></pre>
  
  <p>I'm using RStudio 0.97.332 and R version 2.15.3.</p>
  
  <p>What could be the problem?</p>