Fix WebTranslateIt::AutoFetch

This pull requests fixes two issues with `WebTranslateIt::AutoFetch`.

There was a call to `I18n.lang` in `WebTranslateIt.fetch_translations`, but the `I18n.lang` method does not exist. I replaced it with `I18n.locale`, but am a little unsure if that makes sense.

The other issue this pull request fixes, is that requests to `/assets/application.js?body=1` would fetch the translations… which makes things pretty slow.