Core: Update tested jsdom, drop obsolete workarounds

The latest version supporting Node.js is 3.1.2; some workarounds are not needed
for this version. For example, in jsdom 3.1.2 a document created via
document.implementation.createHTMLDocument( "" ) has a body.

Fixes gh-2153
Closes gh-2154

package.json

@@ -42,7 +42,7 @@
     "grunt-jsonlint": "1.0.4",
     "grunt-npmcopy": "0.1.0",
     "gzip-js": "0.3.2",
-    "jsdom": "1.5.0",
+    "jsdom": "3.1.2",
     "load-grunt-tasks": "1.0.0",
     "native-promise-only": "0.7.8-a",
     "npm": "2.1.12",