Scss-lint grunt & config update

- [new] Update grunt-scss-lint to latest version, bringing colouring to the terminal output
- [new] Disable enforcement of a single rule declaration per Sass file (gets rid of alerts like *"Merge root rule `.blog__head-background--default` with identical rule on line 207"*) /cc @phamann
- ~~[temporary fix] `grunt validate:sass` works with the locally installed scss-lint gem (instead of relying on bundler which prevented it from running)~~
- [new] `grunt validate` lints Sass as well as JS and CSS
- **Update Thursday 8th** [new] Update Sass to latest version

PS: I am currently writing a post about this: ["Improving Your Sass Code Quality With scss-lint"](https://gist.github.com/kaelig/7da58c0a03c2b4df248e).

![ ](http://media.giphy.com/media/LCfmgllXiNfAk/giphy.gif)