Updates firefox javascript version detection to work with FF > 12

### RM #6994

### Verified with browsershots:

Set 1: http://browsershots.org/http://jsbin.com/uleher/17
Set 2: http://browsershots.org/websites/906333970f0137608aa0759e0f1871e7/9150463

### Manual spot checking:
1. Copy the entire source of `lib/rex/exploitation/javascriptosdetect.js`
2. Open your version of Firefox, press cmd/ctrl-option-k (or Tools->Web Developer->Web Console).
3. Paste the js into the console and hit enter. Run the following console cmd: `os_detect.getVersion()`, and look at the `ua_version` attribute.