title: error setting certificate verify locations, install_github
body: <p>I am trying to install a package from github, but I keep getting an error when I
  use <code>install_github</code>.</p>
  
  <pre>
  library(devtools)
  install_github(repo="swirl", username="ncarchedi")
  Installing github repo(s) swirl/master from ncarchedi
  Downloading swirl.zip from https://github.com/ncarchedi/swirl/archive/master.zip
  Error in function (type, msg, asError = TRUE)  : 
    error setting certificate verify locations:
    CAfile: /Library/Frameworks/R.framework/Versions/3.0/Resources/library/RCurl/CurlSSL/cacert.pem
    CApath: none
  </pre>
  
  <p>I have tried to install several different packages
  (including an updated version of ggmap), and I always get the same error. It
  must have something to do with RCurl, but I don't understand what the problem
  is, or how to fix it. I have devtools (Version 1.3) and RCurl
  (Version 1.95-4.1). I am running R version 3.0.1 ("Good Sport") on mac.</p>
  
  <p>Results from <code>sessionInfo()</code></p>
  
  <pre>
  sessionInfo()
  R version 3.0.1 (2013-05-16)
  Platform: x86_64-apple-darwin10.8.0 (64-bit)
  
  locale:
  [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] RCurl_1.95-4.1 bitops_1.0-5   devtools_1.3  
  
  loaded via a namespace (and not attached):
  [1] digest_0.6.3   evaluate_0.4.3 httr_0.2       memoise_0.1    parallel_3.0.1  stringr_0.6.2 
  [7] tools_3.0.1    whisker_0.3-2
  </pre>