title: error: could not find function install_github for R version 2.15.2
body: <p>I'm having multiple problems with R right now but I want to start asking one of the most fundamental questions.</p>
  
  <p>I want to install GitHub files into R, but for some reason the <code>install_github</code> function doesn't seem to exist. For example, when I type:</p>
  
  <pre><code>install_github("devtools")
  </code></pre>
  
  <p>I get</p>
  
  <pre><code>error: could not find function install_github
  </code></pre>
  
  <p>The <code>install_packages</code> function worked perfectly fine. How can I solve this problem?</p>
  
  <p>To add, I want to ask whether there is a way to upgrade R, since version 2.15.2 doesn't seem to be compatible for most of the packages I want to work with.</p>
  
  <p>I'm currently using Linux version 3.6.11-1 RedHat 4.7.2-2 fedora linux 17.0 x86-64.</p>
  
  <p>I checked the CRAN website but they seemed to have the most unupdated versions of R (if that is even possible) that dates all the way back to '09. I would seriously love to update myself from this old version of R. Any advice on this too?</p>