remove stray `?` in webpack.config.js

lib/pipeline/webpack.config.js

@@ -36,7 +36,7 @@ try {
 }
 
 const entry = Object.keys(embarkAssets)
-  .filter(key => key.match(/\.js?$/))
+  .filter(key => key.match(/\.js$/))
   .reduce((obj, key) => {
     // webpack entry paths should start with './' if they're relative to the
     // webpack context; embark.json "app" keys correspond to lists of .js