chore(package): update phantomjs-prebuilt to version 2.1.8

https://greenkeeper.io/

package.json

@@ -53,7 +53,7 @@
     "karma-phantomjs-launcher": "^1.0.1",
     "karma-xml2js-preprocessor": "^0.0.3",
     "mocha": "^3.0.1",
-    "phantomjs-prebuilt": "^2.1.7",
+    "phantomjs-prebuilt": "^2.1.8",
     "tsify": "^1.0.2",
     "tslint": "3.8.0",
     "typescript": "^1.8.10",