title: having trouble getting devtools::install_github() to work in R on Win 7 64bit machine
body: <p>each time I try to run install_github()  from RStudio I get something like this:</p>
  
  <pre class="lang-r prettyprint-override"><code>devtools::install_github("r-lib/devtools")
  </code></pre>
  
  <pre><code>Downloading GitHub repo r-lib/devtools@master
  Error in utils::download.file(url, path, method = download_method(), quiet = quiet,  : 
    cannot open URL 'https://api.github.com/repos/r-lib/devtools/tarball/master'*
  </code></pre>
  
  <p>I tried the work around described in <a href="https://github.com/r-lib/devtools/issues/1772" rel="nofollow noreferrer">https://github.com/r-lib/devtools/issues/1772</a></p>
  
  <p>and this is what I get:</p>
  
  <pre class="lang-r prettyprint-override"><code>assignInNamespace("version_info", c(devtools::version_info, list("3.5" = list(version_min = "3.3.0", version_max = "99.99.99", path = "bin"))), "devtools")
  </code></pre>
  
  <pre><code>Error in bindingIsLocked(x, ns) : no binding for "version_info"
  </code></pre>
  
  <pre class="lang-r prettyprint-override"><code>find_rtools() # is TRUE now
  </code></pre>
  
  <pre><code>Error in find_rtools() : could not find function "find_rtools"*
  </code></pre>
  
  <p>Please suggest a workaorund to get <code>devtools::install_github()</code> to work on my Win PC.
  thank you</p>