Handle Gist URLs for past commits

Past commits are represented as {original_id}/{version}

Here's a past commit of a gist I had tried in RequireBin:
https://gist.github.com/BigBlueHat/f47e6cfda27afccc03be/c02328cde004eda920a8c39a36858b2add7f702d

I attempted: [`?gist=f47e6cfda27afccc03be/c02328cde004eda920a8c39a36858b2add7f702d`](http://requirebin.com/?gist=f47e6cfda27afccc03be/c02328cde004eda920a8c39a36858b2add7f702d) which threw this error:
```
{"error":"no index.js in this gist","json":{"message":"Not Found","documentation_url":"https://developer.github.com/v3"}}
```
...because it was treating the first part (`f47e6cfda27afccc03be`) as the user.

Adding the user to the mix: [`?gist=BigBlueHat/f47e6cfda27afccc03be/c02328cde004eda920a8c39a36858b2add7f702d`](http://requirebin.com/?gist=BigBlueHat/f47e6cfda27afccc03be/c02328cde004eda920a8c39a36858b2add7f702d) results in the latest version of the Gist being returned--not the requested past commit.

This patch should handle both cases--despite the `user` variable not always being a username (re-name it if you must :wink: ).