Fix for unquoting of sass arguments required for sass 3.4.0+

For this to work my pull request for `dargs` need to be merged first:
https://github.com/sindresorhus/dargs/pull/9

It enabled users to unquote arguments to `sass`. `grunt-contrib-sass` 0.8.0 and higher do not work correctly with sass 3.4.0 and higher due to quoting. This at least occurs under Linux and OSX.