chore(package): update bufferutil to version 4.0.0

Author: greenkeeper[bot]

package.json

@@ -52,7 +52,7 @@
     "webtorrent-fixtures": "^1.3.0"
   },
   "optionalDependencies": {
-    "bufferutil": "^3.0.0"
+    "bufferutil": "^4.0.0"
   },
   "keywords": [
     "bittorrent",