Rename ol.Observable#dispatchChangeEvent() to #changed()

Since dispatchChangeEvent does not only dispatch a change event, but
also increases the revision counter, the method name should reflect
this.

src/ol/source/clustersource.js

@@ -81,7 +81,7 @@ ol.source.Cluster.prototype.onSourceChange_ = function() {
   this.clear();
   this.cluster_();
   this.addFeatures(this.features_);
-  this.dispatchChangeEvent();
+  this.changed();
 };