Use unreleased version of contextify which fixes nan dependency

From this PR: brianmcd/contextify#181

package.json

@@ -57,7 +57,7 @@
   "repository": "tmpvar/jsdom",
   "dependencies": {
     "browser-request": ">= 0.3.1 < 0.4.0",
-    "contextify": ">= 0.1.9 < 0.2.0",
+    "contextify": "git://github.com/rvagg/contextify/#nan2",
     "cssom": ">= 0.3.0 < 0.4.0",
     "cssstyle": ">= 0.2.21 < 0.3.0",
     "htmlparser2": ">= 3.7.3 < 4.0.0",