Upgrade libxmljs dependency to 0.18. Fix #1

Author: aarranz

package.json

@@ -24,7 +24,7 @@
     "sinon": "^1.17.3"
   },
   "dependencies": {
-    "libxmljs": "^0.17.1",
+    "libxmljs": "^0.18.4",
     "sync-request": "^3.0.0"
   }
 }