Pretty output with Slim templates

[webinetics]

How do I get Middleman to create "pretty" (nested) output when using
Slim templates?

[Thomas Reynolds]

In your config.rb:

  set :slim, :pretty => true

-Thomas

[Isa Goksu]

This doesn't seem to work.. Neither with 'middleman server' nor after
building the site. What am I doing wrong? My config.rb is below:

set :slim, :pretty => true

use Rack::Codehighlighter, :ultraviolet, :theme => "idle", :element =>
"pre", :pattern => /\A::([-_+\w]+)::\s*/
configure :build do
activate :minify_css
activate :minify_javascript
activate :cache_buster
activate :directory_indexes
end