title: Using devtools' github_install without force = TRUE
body: <p>I use devtools and after I updated to R 3.3.1 I have this message every time I install something via Github.</p>
  
  <pre><code>Skipping install of 'PACKAGE' from a github remote, the SHA1 (123456) has not changed since last install.
    Use `force = TRUE` to force installation
  </code></pre>
  
  <p>Anybody had this issue too?</p>