Prevent webpack from hanging during staging

Staging was hanging indefinitely when it started optimizing assets
(part of the `npm run build` postinstall script).

I didn't investigate fully, but it seems to be related to Node's poor
grasp of system resources[1,2,3].

One suspect was identified in the build output:

```
> npm run build (node:46) MaxListenersExceededWarning: Possible
  EventEmitter memory leak detected. 11 SIGINT listeners added. Use
  emitter.setMaxListeners() to increase limit
```

[1] webpack/webpack#2012 (comment)
[2] webpack/webpack#4558
[3] webpack-contrib/sass-loader#100

package.json

@@ -259,4 +259,3 @@
     "reporter": "lcov"
   }
 }
-