title: Google API key error in ggmap even when specifying other source
body: <p>I've started using the package <code>ggmap</code> after a while again, now I'm encountering a problem when I try to fetch a map using the function <code>get_map</code>. No matter which source I specify, I always get the error</p>
  
  <pre><code>Error: Google now requires an API key.
     See ?register_google for details.
  </code></pre>
  
  <p>although I'm not sure why a call like</p>
  
  <pre><code>get_map(location = "texas", zoom = 6, source = "stamen")
  </code></pre>
  
  <p>Would need an API call to google. Has anyone experience something similar and is there a known workaround?</p>