Modified gists.js to account for the correct Dropbox base URL

I couldn't get links to AsciiDoc files in Dropbox to work. For example, this GraphGist failed https://www.dropbox.com/s/edrw7kbhetwyzza/gistcheck.adoc failed with the error message 

```
Error
Something went wrong fetching the GraphGist "https%3A%2F%2Fwww.dropbox.com%2Fs%2Fedrw7kbhetwyzza%2Fgistcheck.adoc":
```

This appears to be because the `DROPBOX_BASE_URL` in [gists.js](https://github.com/neo4j-contrib/graphgist/blob/master/js/gist.js) is `https://dl.dropboxusercontent.com/u/` but the URLs I'm getting from Dropbox these days are prefixed with `https://www.dropbox.com/s/`