title: ggmap Error: GeomRasterAnn was built with an incompatible version of ggproto
body: <p>I'm using ggmap, and got the error below:</p>
  
  <pre><code>Error: GeomRasterAnn was built with an incompatible version of ggproto.
  Please reinstall the package that provides this extension.
  </code></pre>
  
  <p>I've installed the latest version of both ggmap(2.6.1) and ggplot2(2.2.0), but still got the same error.</p>