Update Sass dependency to 3.2.5

Sass 3.2.0.alpha.93 doesn't support variable arguments, so relying on Compass to install Sass results in syntax errors inside Compass, eg when importing [css3](https://github.com/chriseppstein/compass/blob/1724d70bd9c77aff31dc1c6e596e8b476fc7a656/frameworks/compass/stylesheets/compass/css3/_shared.scss#L220).