SpiderMonkey nightly builds

[spidermonkey/get-latest-version.js](https://github.com/GoogleChromeLabs/jsvu/blob/master/engines/spidermonkey/get-latest-version.js) currently only supports Beta builds, but it should also be possible to download Nightly builds:

- Nightly binaries are available at .
- The build-id can either be retrieved by scraping and searching for one of these files:
- https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/firefox-70.0a1.en-US.linux-x86_64.buildhub.json
- https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/firefox-70.0a1.en-US.linux-x86_64.json
- https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/firefox-70.0a1.en-US.linux-x86_64.txt
- https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/firefox-70.0a1.en-US.linux-x86_64_info.txt
- Or a bit more fancy through [buildhub2](https://buildhub2.readthedocs.io/en/latest/project.html):
- For example for linux64 nightly builds:
```sh
curl -s -X POST https://buildhub.moz.tools/api/search \
-d '{"size": 1, "sort": {"build.id": "desc"}, "query": {"bool": {"must": [{"term": {"source.product": "firefox"}}, {"term": {"source.tree": "mozilla-central"}}, {"term": {"target.channel": "nightly"}}, {"term": {"target.platform": "linux-x86_64"}}]}}}'
```