Recent changes to upstream (awestruct and/or tilt) mean that asciidoctor is now required as a dependency.

When I tried to use "awestruct -d" to run the Ceylon wevsite from a fresh checkout with a fresh RVM instance, it failed saying that that asciidoctor was missing (see https://groups.google.com/d/msg/ceylon-dev/bDsdYrdH8Jg/SpnxGWq8jn4J)

Uncommenting the line in the Gemfile cures this.

Apparently, when awestruct bumped to 0.5.0, a number of its dependencies became optional.  And at the same time, they switched to tilt for a lot of file rendering, and it seems that tilt also depends on asciidoctor.  Somewhere in this mess of upstream changes, it appears that we've picked up an implicit dependency that has to be fulfilled explicitly.

(I'm not convinced that "the cure" in this commit is not just papering over the cracks of a problem that ought to be fixed ... or at least documented better ... in awestruct.  I'll leave it to experts to judge that ...)

References:

* http://awestruct.org/getting_started/ - See "Setting up your dependencies"
* https://github.com/awestruct/awestruct/wiki/Migrating-to-Awestruct-0.5.0 - see "Tilt and dependencies"