Be more precise about the origin of webcomponentsjs

Author: bartolom

bower.json

@@ -25,7 +25,7 @@
     "url": "https://github.com/Polymer/polymer.git"
   },
   "dependencies": {
-    "webcomponentsjs": "^0.7.20"
+    "webcomponentsjs": "webcomponents/webcomponentsjs#^0.7.20"
   },
   "devDependencies": {
     "web-component-tester": "*",