Wait for final stream to finish

When building the bundle, wait for the final stream to finish writing
output, rather than the intermediate stream.

scripts/gulp/create-bundle.js

@@ -221,7 +221,7 @@ module.exports = function createBundle(config, buildOpts) {
       stream = stream.pipe(minifyStream());
     }
 
-    stream.pipe(exorcist(sourcemapPath)).pipe(output);
+    stream = stream.pipe(exorcist(sourcemapPath)).pipe(output);
     return streamFinished(stream);
   }