Update sass / dependencies

Hey there,

we currently working on a project which requires Sass version 3.3.x, we use Susy 2.x for the layouts. `wagon` / `mounter` currently requires Sass version 3.2.x, so we updated both.

We removed `sprockets` and `sprockets-sass` from the wagon gemspec, because `locomotivecms_mounter` already defined them.

We tested the changes, we could view the website locally (`wagon server`) and push the site to an engine (latest version).