Use protocol-relative URL for Bing metadata.

The [URI standard](http://tools.ietf.org/html/rfc3986#section-4.2)
allows for URI without a scheme.  Where http is used to load the app,
using a schemeless URL for the Bing metadata loads a script using http
- and the same goes for https.

src/ol/source/bingmaps.js

@@ -55,7 +55,7 @@ ol.source.BingMaps = function(bingMapsOptions) {
   this.ready_ = false;
 
   var uri = new goog.Uri(
-      'http://dev.virtualearth.net/REST/v1/Imagery/Metadata/' +
+      '//dev.virtualearth.net/REST/v1/Imagery/Metadata/' +
       bingMapsOptions.style);
   var jsonp = new goog.net.Jsonp(uri, 'jsonp');
   jsonp.send({