Fix Proofer test

Proofer crashes on the [build](https://travis-ci.org/IIIF/iiif.io/builds/25170819):

```
htmlproof 0.6.7 | Error:  No such file or directory @ rb_sysopen - /home/travis/build/IIIF/iiif.io/_site/api/image/index.html
```

I will investigate.