fix: Use the default merge for webpack config.

The smart merge feature has been dropped from the webpack-merge library
that we were using. Use the basic merge feature instead since we're not
actually doing anything too complicated.

Also don't return null from the WorkerConfig as it can't be correctly
merged.

webpack.dev.config.js

@@ -9,7 +9,7 @@ var _ = require('underscore');
 
 var commonConfig = require('./webpack.common.config.js');
 
-module.exports = _.values(Merge.smart(commonConfig, {
+module.exports = _.values(Merge.merge(commonConfig, {
     web: {
         output: {
             filename: '[name].js'