Add required node version

[damiankk]

The project doesn't have required node.js version, so it has to be added to the package.json
@michalmikolajczyk told me that he uses 9.4 version

{ 
  "engines" : {
    "node" : ">=9.4"
  }
}

[michalmikolajczyk]

Nicely done @damiankk !