display current build on editor download button

src/site/js/editor-version.js

+$(document).ready(function() {
+  var displayIntegrationVersion = function() {
+    fetchEditorVersion('integration');
+  };
+
+  var updatePlaceholders = function(channel, version) {
+    $('.editor-build-rev-' + channel).each(function(index, elem) {
+      elem.innerHTML = version;
+    });
+  };
+
+  var fetchEditorVersion = function(buildType) {
+    $.ajax({
+      type: "GET",
+      url: 'http://dart-editor-archive-' + buildType + '.commondatastorage.googleapis.com/latest/VERSION',
+      dataType: "json",
+      success: function(data) {
+        updatePlaceholders(buildType, data['revision']);
+      },
+      error: function(xhr, textStatus, errorThrown) {
+        console.log(textStatus);
+      }
+    })
+  };
+
+  displayIntegrationVersion();
+});