title: Issue installing R package from github using devtools
body: <p>In similar fashion to the following question:</p>
  
  <p><a href="https://stackoverflow.com/questions/12829100/error-when-using-install-github-exdir-does-not-exist?rq=1">Error when using install_github: &#39;exdir&#39; does not exist</a></p>
  
  <p>I cant seem to install packages from github using the <code>devtools</code> package, getting the following error on all packages I've tried (slidify, rCharts, rga, magrittr etc...):</p>
  
  <pre><code>Error in unzip(src, list = TRUE) : 
    zip file 'C:/Users/.../Rtemp\RtmpGIlGBA/master.zip' cannot be opened
  In addition: Warning messages:
  1: In unzip(src, exdir = target, unzip = getOption("unzip")) :
      error 1 in extracting from zip file
  </code></pre>
  
  <p>I've changed the temporary directory to somewhere writeable in case this was the problem:</p>
  
  <pre><code>    write("TMP = 'C:/Users/.../Rtemp' \n
             TMPDIR = 'C:/Users/.../Rtemp' \n
             TEMP = 'C:/Users/.../Rtemp'", 
              append = TRUE,
              file = file.path(Sys.getenv('R_USER'), '.Renviron'))
      readRenviron(".Renviron")
  </code></pre>
  
  <p>But to no avail. I have also <code>setwd()</code> to somewhere else and checked whether there is an issue with <code>unzip()</code> by getting the zip file from github directly:</p>
  
  <pre><code>temp &lt;- tempfile()
  download.file("https://github.com/ramnathv/slidifyLibraries/archive/master.zip",temp)
  unzip(temp)
  </code></pre>
  
  <p>This appears to work fine, and I've since installed the packages using <code>build()</code> etc... but it is <strong>not</strong> as handy as the <code>install_github()</code>. Any ideas?</p>
  
  <p>Here is my <code>sessionInfo()</code> :</p>
  
  <pre><code>R version 3.1.1 (2014-07-10)
  Platform: x86_64-w64-mingw32/x64 (64-bit)
  
  locale:
  [1] LC_COLLATE=English_Ireland.1252  LC_CTYPE=English_Ireland.1252    LC_MONETARY=English_Ireland.1252 LC_NUMERIC=C                    
  [5] LC_TIME=English_Ireland.1252    
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] devtools_1.5 httr_0.3    
  
  loaded via a namespace (and not attached):
  [1] digest_0.6.4   evaluate_0.5.5 memoise_0.2.1  parallel_3.1.1 RCurl_1.95-4.2 stringr_0.6.2  tools_3.1.1    whisker_0.3-2 
  Warning message:
  closing unused connection 3 (https://github.com/ramnathv/rCharts/archive/master.zip)
  </code></pre>