Integrate middleman-syntax based syntax highlighting + upgrades

Had this on my OS hit list for some [Middleman](http://middlemanapp.com) outreach.  Hope the changes aren't too sweeping.

### Big Changes
- Upgraded Middleman to `~> 3.2`.
- Replaced [Ultraviolet](https://github.com/spox/ultraviolet) with [Middleman's supported syntax highlighting plugin](https://github.com/middleman/middleman-syntax) (which uses [Rouge](https://github.com/jayferd/rouge) under the hood). This will change up how the syntax highlighting structure looks but my Tab Switch Tests™ indicates that you won't notice a thing.
- The commit that kicked off the whole pull request was https://github.com/middleman/middleman-syntax/commit/8466a2871942e64cd44b512b073f428d01dd2568 which should clean up `haml_filters.rb` (by it simply becoming obsolete) and is why `middleman-syntax` is tracking head.
- This commit will be very annoying for any translations going on or in progress :unamused: sorry.

### Subtle Changes
- Rouge uses [Pygments compatible stylesheets](http://pygments.org) I've done my best to migrate the sunburst theme but It may need more tweaking. My recommendation would be to have a look at the [built in stylesheets](https://github.com/middleman/middleman-syntax#css) and see if one takes your fancy?
- I've enabled `activate :minify_css` for builds which shouldn't cause too much pain but just a heads up to check the css formatting after a `middleman build`.
- General tidy up of `config.rb` such as popping `activate :livereload` to be a `development` only plugin.

### Gripes.
- Only one, which seems to be indentation issues on syntax blocks made obvious on the [Groups page](http://bundler.io/v1.5/groups.html). I've had a quick play around at trying to make all the moving pieces play well and haven't gotten anywhere. Even with changing to `middleman-syntax` you'll still see this problem :-1:.

Enjoy :+1: