1022767 use json l10n manifests

https://bugzilla.mozilla.org/show_bug.cgi?id=1022767
So, I followed l20n master - https://github.com/l20n/l20n.js/blob/master/lib/l20n/context.js#L556 - since Stas is updating the ctx API, I think it's good for now and we will update both, l20n master and gaia master API's soon.