Avoid unexpected "ws" warnings generated by webpack...

WARNING in /Users/moox/Sync/Development/phenomic/node_modules/ws/lib/BufferUtil.js
Module not found: Error: Can't resolve 'bufferutil' in '/Users/moox/Sync/Development/phenomic/node_modules/ws/lib'

WARNING in /Users/moox/Sync/Development/phenomic/node_modules/ws/lib/Validation.js
Module not found: Error: Can't resolve 'utf-8-validate' in '/Users/moox/Sync/Development/phenomic/node_modules/ws/lib'

Ref websockets/ws#1220

packages/plugin-bundler-webpack/src/WebpackServerConfigModifier.js

@@ -23,6 +23,10 @@ const defaultExternals = [
   "react",
   "react-dom",
 
+  // https://github.com/websockets/ws/issues/1220
+  "bufferutil",
+  "utf-8-validate",
+
   /^apollo(\/.*)?/,
   /^aphrodite(\/.*)?/,
   /^emotion(\/.*)?/,