Prefer arrow functions (#2344)

scripts/build-node.js

@@ -29,7 +29,7 @@ config.entry = "./src/index-node";
 // By default, webpack is going to replace the require of the canvas.node file
 // to just a string with the path of the canvas.node file. We need to tell
 // webpack to avoid rewriting that dependency.
-config.externals = function (context, request, callback) {
+config.externals = (context, request, callback) => {
   if (/\.node$/.test(request)) {
     return callback(
       null,