Add compatibility shim for Node < 8

Author: Leonidas-from-XIV

package-lock.json

@@ -75,7 +75,8 @@
   },
   "dependencies": {
     "sax": ">=0.6.0",
-    "xmlbuilder": "~11.0.0"
+    "xmlbuilder": "~11.0.0",
+    "util.promisify": "~1.0.0"
   },
   "devDependencies": {
     "coffee-script": ">=1.10.0",