Remove LESS support in `package.json` and `Gruntfile.js`, add `grunt-sass` and `grunt-jsvalidate` for future usage, and update `cssmin` and `uglify` to latest versions.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8550 cdf35c40-ae34-48e0-9cc9-0c9da1808c22