Bump amber dep to >=0.21.0.

B/c: uses `Platform class >> includesGlobal:`.

bower.json

@@ -20,7 +20,7 @@
     ],
     "private": false,
     "dependencies": {
-        "amber": ">=0.19.0",
+        "amber": ">=0.21.0",
         "amber-contrib-web": ">=0.5.0",
         "jquery-ui": ">=1.9.2",
         "codemirror": "^5.25.2",