title: devtools::install_github fails with CA cert error
body: <p>When I try to call install_github, I get the following error (not just for this package, but for all github packages):</p>
  
  <pre><code>&gt; install_github('ramnathv/slidify')
  Downloading github repo ramnathv/slidify@master
  Error in curl::curl_fetch_memory(url, handle = handle) :
  Problem with the SSL CA cert (path? access rights?)
  </code></pre>
  
  <p>But if I use RCurl directly to access github with ssl, I don't get any problem:</p>
  
  <pre><code>&gt;  x &lt;- getBinaryURL(
            url='https://github.com/ramnathv/slidify/archive/master.zip',
            followlocation=1L
            )
  </code></pre>
  
  <p>works with no errors, so RCurl can verify the SSL certificate properly and download the archive file.</p>
  
  <pre><code>&gt; sessionInfo()
  R version 3.2.1 (2015-06-18)
  Platform: x86_64-pc-linux-gnu (64-bit)
  Running under: Debian GNU/Linux 8 (jessie)
  
  locale:
   [1] LC_CTYPE=en_US.utf8       LC_NUMERIC=C             
   [3] LC_TIME=en_US.utf8        LC_COLLATE=en_US.utf8    
   [5] LC_MONETARY=en_US.utf8    LC_MESSAGES=en_US.utf8   
   [7] LC_PAPER=en_US.utf8       LC_NAME=C                
   [9] LC_ADDRESS=C              LC_TELEPHONE=C           
   [11] LC_MEASUREMENT=en_US.utf8 LC_IDENTIFICATION=C      
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] RCurl_1.95-4.7 bitops_1.0-6   devtools_1.8.0
  
  loaded via a namespace (and not attached):
   [1] httr_1.0.0      R6_2.1.0        magrittr_1.5    rversions_1.0.2
   [5] tools_3.2.1     curl_0.9.1      Rcpp_0.12.0     memoise_0.2.1  
   [9] xml2_0.1.1      stringi_0.5-5   git2r_0.10.1    stringr_1.0.0  
   [13] digest_0.6.8   
  </code></pre>
  
  <p>And</p>
  
  <pre><code>&gt; curlVersion()
  
  $age
  [1] 3
  
  $version
  [1] "7.38.0"
  
  $vesion_num
  [1] 468480
  
  $host
  [1] "x86_64-pc-linux-gnu"
  
  $features
     ipv6         ssl        libz        ntlm   asynchdns      spnego 
        1           4           8          16         128         256 
     largefile         idn tlsauth_srp     ntlm_wb 
      512        1024       16384       32768 
  
  $ssl_version
  [1] "OpenSSL/1.0.1k"
  
  $ssl_version_num
  [1] 0
  
  $libz_version
  [1] "1.2.8"
  
  $protocols
  [1] "dict"   "file"   "ftp"    "ftps"   "gopher" "http"   "https"  "imap"  
  [9] "imaps"  "ldap"   "ldaps"  "pop3"   "pop3s"  "rtmp"   "rtsp"   "scp"   
  [17] "sftp"   "smtp"   "smtps"  "telnet" "tftp"  
  
  $ares
  [1] ""
  
  $ares_num
  [1] 0
  
  $libidn
  [1] "1.29"
  </code></pre>
  
  <p>If I use <code>httr::set_config( httr::config( ssl_verifypeer = 0L ) )</code> then I can successfully run <code>install_github</code> but I would prefer to actually check ssl certificates. </p>
  
  <p>Can anyone offer a solution?</p>