fix gist API fallback

[Rich-Harris]

looks like I broke this during the Sapper rewrite

[Conduitry]

Possibly all this needs is a change to https://github.com/sveltejs/svelte.technology/blob/master/routes/api/gists/[id].js to add

let credentials = {
	id: process.env.client_id,
	secret: process.env.client_secret
};

[Rich-Harris]

I suspect that was the correct diagnosis, though it's rendered moot by #224 — will close this