Fix URL tags in _data without custom ruby code

[jacquerie]

As @RaoOfPhysics asked, here's a cherry-pick of @ibab's commit that solved the issue. It appears to work locally, let's see if GitHub agrees...

[betatim]

Thanks a lot for this! @ibab do you know what setup/command we need locally to reproduce the restrictions of github?

The inner nerd in me feels nervous with automated testing :sweat: Knowing the command(s) needed could form the basis for starting a travis.yml in a new PR.

[ibab]

Thanks for cherry-picking the commit! @betatim: You can test the restricted build by appending --safe as in jekyll serve --safe

[betatim]

Merge?