Use new AppForClient in derby@4 if present (#29)

derby@4 moves client-only logic into a new AppForClient subclass, so the derby-webpack override of _views() needs to happen on the subclass. This falls back to using App for backwards compatibility.

lib/browser.js

@@ -1,5 +1,7 @@
 if (process.title === 'browser') {
-  const App = require('derby/App').App;
+  const AppModule = require('derby/App');
+  // AppForClient introduced in derby@4
+  const App = AppModule.AppForClient || AppModule.App;
 
   App.prototype._views = function () {
     const appName = this.name;