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).