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.