Update grunt-contrib-sass to 0.6.0

This release ignores files where filename have leading underscore.

https://github.com/gruntjs/grunt-contrib-sass/blob/df01db3664500fa8b83afb015541c01e7b5804ca/CHANGELOG
Merge pull request #74 from alphagov/update-grunt-contrib-sass-0-6-0

Update grunt-contrib-sass to 0.6.0