title: Error while using install_github | devtools | timeout issue
body: <p>I'm facing this issue when i try to download and install a package from git-hub using devtools. </p>
  
  <p>R Version : 3.1.3 (64 bit)</p>
  
  <pre><code>&gt; install_github("hadley/httr")
  Downloading github repo hadley/httr@master
  Error in curl::curl_fetch_memory(url, handle = handle) : 
    Timeout was reached
  
  &gt; traceback()
  13: .Call(R_curl_fetch_memory, url, handle)
  12: curl::curl_fetch_memory(url, handle = handle)
  11: request_fetch.write_memory(req$output, req$url, handle)
  10: request_fetch(req$output, req$url, handle)
  9: request_perform(req, hu$handle$handle)
  8: httr::HEAD(src_submodules, , auth)
  7: github_has_remotes(x, auth)
  6: remote_download.github_remote(remote, quiet = quiet)
  5: remote_download(remote, quiet = quiet)
  4: FUN(X[[1L]], ...)
  3: vapply(remotes, install_remote, ..., FUN.VALUE = logical(1))
  2: install_remotes(remotes, ...)
  1: install_github("hadley/httr")
  </code></pre>
  
  <p>Edits: </p>
  
  <p>I am using Revolution R Enterprise 7.4 with a Rstudio client.</p>