Add node-util as dep

Author: s1na

package.json

@@ -38,7 +38,8 @@
     "level-mem": "^3.0.1",
     "lru-cache": "^5.1.1",
     "safe-buffer": "^5.1.2",
-    "semaphore": "^1.1.0"
+    "semaphore": "^1.1.0",
+    "util": "^0.11.1"
   },
   "devDependencies": {
     "@babel/cli": "^7.2.3",