Adapt sources to new signature for ol.Attribution

src/ol/source/stamensource.js

@@ -114,8 +114,10 @@ goog.inherits(ol.source.Stamen, ol.source.XYZ);
  * @const {Array.<ol.Attribution>}
  */
 ol.source.Stamen.ATTRIBUTIONS = [
-  new ol.Attribution(
-      'Map tiles by <a href="http://stamen.com/">Stamen Design</a>, under ' +
-      '<a href="http://creativecommons.org/licenses/by/3.0/">CC BY 3.0</a>.'),
+  new ol.Attribution({
+    html: 'Map tiles by <a href="http://stamen.com/">Stamen Design</a>, ' +
+        'under <a href="http://creativecommons.org/licenses/by/3.0/">CC BY' +
+        ' 3.0</a>.'
+  }),
   ol.source.OSM.DATA_ATTRIBUTION
 ];