Fix broken gist loading

Fixes https://github.com/mapbox/geojson.io/issues/385

Requests for [a list of gists](https://developer.github.com/v3/gists/#list-gists) (e.g. paginated gists) do not contain a 'content' field, a second request needs to get the [selected gist](https://developer.github.com/v3/gists/#get-a-single-gist) on click to get the content of the gist.