Greenkeeper builds fail due to our caching method for node_modules

Quoting https://github.com/code-corps/code-corps-ember/pull/1381#issuecomment-330734655
>Something in the way we're doing caching of our `node_modules` is broken here.

>You can see when the restore tries to happen that it fails:

```
No cache is found for key: dependency-cache-+kGUB3JuQQmksW+LUxPJerNDS37R6sDf8daIJCOw8FY=-8HTuZ8asTuy34o5mEvgz3dI7FaVDhbZL6IKcXolKMvM=
```

>I'm not sure which part of this key is broken precisely or why, but it needs investigating. This will happen with all future Greenkeeper builds if not found and fixed.

先重現一次 Greenkeeper 建置，並檢查負責快取 node_modules 的設定，尤其是 dependency-cache 金鑰如何產生和還原。當快取成功還原，且未來的 Greenkeeper 建置不再因找不到快取而失敗時，該 issue 就完成了。

由索引模型根據 Issue 內容生成。