Remove bundled rkelly, add rkelly-remixed to the Gemfile

rkelly-remixed is a faster fork of rkelly that is more frequently updated nowadays. With the new gem, jsobfu obfuscates os.js about twice as fast on my dev environment.

### Verification:

- [ ] Specs pass
- [ ] Add `alert(JSON.stringify(window.os_detect.getVersion()));` to the bottom of `data/js/detect/os.js`
- [ ] Run browser_auto_pwn multiple times against Chrome and another browser that is not an exploit target
- [ ] You should get a popup with the right version info.
- [ ] Test a browser exploit that uses the BrowserExploitServer mixin (`firefox_proto_crmfrequest` will work against FF 5-15)