Rouge dependency doesn’t correctly syntax highlight ES6 string interpolation

[kapowaz]

The rouge dependency set by github-pages is at v1.11.1, which doesn’t include more recent fixes that enable supporting ES6 syntax highlighting. This means that for example:

return `${firstName} ${lastName}`;

…will highlight the backticks as errors.

[benbalter]

Thanks @kapowaz. This is dependent and somewhat duplicative of jekyll/jekyll#5230. Once Rouge 2.x is supported upstream, we can update the dependency here.