Using [semver](https://github.com/isaacs/node-semver), we can probably use engines field to replace the app ID and versions that are currently baked into the install.rdf with this. Thoughts?

```
"engines": {
  "Firefox": ">19.0.0",
  "Fennec": ">19.0.0"
}
```

@erikvold is this something we could do in AOM? A downside would be, due to our rapidly increase major version number, it wouldn't be possible (AFAIK) to say versions >24 and < 29, for a non-Australis addon for example. But in that case, it'd probably be okay just to do ` < 29`