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.

With my fix arguments are unquoted by default. They can be set to quoted for older sass versions.