chore: pin bufferutil to 4.0.0

Author: greenkeeper[bot]

package.json

@@ -51,7 +51,7 @@
     "webtorrent-fixtures": "^1.3.0"
   },
   "optionalDependencies": {
-    "bufferutil": "^4.0.0",
+    "bufferutil": "4.0.0",
     "utf-8-validate": "^5.0.1"
   },
   "keywords": [