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

manifest.yml

@@ -2,7 +2,7 @@
 applications:
 - name: paas-hackmd
   instances: 1
-  memory: 2G
+  memory: 6G
   buildpack: nodejs_buildpack
   env:
     PGSSLMODE: require

package.json

@@ -9,7 +9,7 @@
     "jsonlint": "find . -not -path './node_modules/*' -type f -name '*.json' -o -type f -name '*.json.example' | while read json; do echo $json ; jq . $json; done",
     "standard": "node ./node_modules/standard/bin/cmd.js",
     "dev": "webpack --config webpack.config.js --progress --colors --watch",
-    "build": "webpack --config webpack.production.js --progress --colors --bail",
+    "build": "node --max-old-space-size=6144 node_modules/webpack/bin/webpack.js --config webpack.production.js --progress --colors --bail",
     "postinstall": "npm run build",
     "start": "node app.js",
     "doctoc": "doctoc --title='# Table of Contents' README.md"