[pkg] Add engines field

Author: lpinca

package.json

@@ -3,6 +3,9 @@
   "version": "4.0.3",
   "description": "WebSocket buffer utils",
   "main": "index.js",
+  "engines": {
+    "node": ">=6.14.2"
+  },
   "scripts": {
     "install": "node-gyp-build",
     "prebuild": "prebuildify --napi --strip",