Fix highlighting (broken for highlight.js >= 8.0.0)

I grabbed docco and the highlight wasn't working on my machine.  I assumed I had a broken dependency or something wrong with my computer, but eventually I realized that it was broken because it grabbed the latest highlight.js, which includes backwards incompatible changes.  The change that caused a problem was:

> We're now prefixing all classes located in CSS classes reference with hljs-, by default, because some class names would collide with other people's stylesheets.

You can see the rest of the release notes at http://highlightjs.org/.

In order to avoid this happening again, I also changed the version constraints to continue grabbing patches and minor releases automatically, but to not automatically do major upgrades.  Since many projects now follow the [Semantic Versioning Specification](http://semver.org/) a major version change indicates an incompatible API change.  This also matches up with nodejitsu's post [package.json dependencies done right](http://blog.nodejitsu.com/package-dependencies-done-right).  [Gemnasium](https://gemnasium.com/) now supports NPM if you want a report to make sure the dependencies don't fall too far behind.