Remove usage of //cesiumjs.org/stk-terrain/world

//cesiumjs.org/stk-terrain/world is an old url that has not been in the
official documentation for some time, but we never officially deprecated
it.  The new cesiumjs.org server is no longer colocated with assets.agi.com,
so the redirect we now provide will have reduced performance.

Apps/Sandcastle/gallery/Interpolation.html

@@ -40,7 +40,7 @@
 
 //Use STK World Terrain
 viewer.terrainProvider = new Cesium.CesiumTerrainProvider({
-    url : '//cesiumjs.org/stk-terrain/world',
+    url : '//assets.agi.com/stk-terrain/world',
     requestWaterMask : true,
     requestVertexNormals : true
 });

CHANGES.md

@@ -2,7 +2,8 @@ Change Log
 ==========
 
 ### 1.11 - 2015-07-01
-
+* Deprecated
+  * The STK World Terrain url `cesiumjs.org/stk-terrain/world` has been deprecated, use `assets.agi.com/stk-terrain/world` instead.  A redirect will be in place until 1.14.
 * Improved the algorithm that `Camera.viewRectangle` uses to select the position of the camera, so that the specified rectangle is now better centered on the screen [#2764](https://github.com/AnalyticalGraphicsInc/cesium/issues/2764).
 * The performance statistics displayed by setting `scene.debugShowFramesPerSecond` to `true` can now be styled using the `cesium-performanceDisplay` CSS classes in `shared.css` [#2779](https://github.com/AnalyticalGraphicsInc/cesium/issues/2779).
 * Fixed a crash when `viewer.zoomTo` or `viewer.flyTo` were called immediately before or during a scene morph [#2775](https://github.com/AnalyticalGraphicsInc/cesium/issues/2775).