Depend on node ~0.8.20

Older versions are incompatible with uglifyjs.

package.json

@@ -23,5 +23,8 @@
     "uglify-js": "~2.2.5",
     "mocha-phantomjs": "~1.1.1",
     "glob": "~3.1.21"
+  },
+  "engines": {
+    "node": "~0.8.20"
   }
 }