Use base_url in snippets

[juhasch]

Fixes issue #1210

[jcb91]

Good job catching this @juhasch :)

For full generality (e.g. someone at some point renames snippets directory, puts it in a subdirectory, etc), we could add 'require' to the requirements list, then use a relative path like requirejs.toUrl('./snippets.json')

But I think this version is fine here, since we don't add any instructions for separate installs anyway.

[juhasch]

Thanks.