Switch to language_sniffer

Github's `linguist` gem was very helpful, but the overhead and number of dependencies for simple language detection is unnecessary. `language_sniffer` is does all that we really
need.

- [x] Passing specs with `language_sniffer` instead of `linguist`
- [x] Restore original JSON detail that was initially lost. (**Edit: Not using this info. Drop it for now**)