Fixing a bug where "grunt default" sass didn't work

The issue was that default ran copy:assets, which ran clean:sass. So later, when default ran compass:dev,
the web/assets/sass files had already been removed
Using the asset function

See https://github.com/knpuniversity/symfonycon-frontend/commit/fe37dc1e8e198f73869052a817b9cab9a998d3d5#commitcomment-5364899
Merge pull request #4 from knpuniversity/assets-function-and-fixes

Assets function and fixes