Upgrade PhantomJS to 2.1.1

Some tests are failing with PhantomJS 1.x and Angular 1.5. See [this AngularJS issue](angular/angular.js#13794).

Author: krtek

package.json

@@ -28,7 +28,7 @@
     "karma-mocha": "^0.2.0",
     "karma-phantomjs-launcher": "^0.2.0",
     "mocha": "^2.2.4",
-    "phantomjs": "^1.9.17"
+    "phantomjs-prebuilt": "^2.1.1"
   },
   "dependencies": {}
 }