Adds feature detection logic for FF 23 and 24 to jsosdetect

### Verification steps:
- [ ] Dump the jsosdetect script to your hd somewhere:

```
msf> irb
[*] Starting IRB shell...
File.write('/Users/joe/Desktop/jsosdetect.js', ::Rex::Exploitation::JavascriptOSDetect.new)
```

- [ ] Download Firefox 22, 23, and 24, one by one run them, open Web Console (cmd-opt-k) and paste the contents of `jsosdetect.js` then run `window.os_detect.getVersion().ua_version`

- [ ] Verify that the correct version is output for 22, 23, and 24.