Build: add SCSS linter (grunt-scss-lint) to Grunt build process.

As part of the companion stylesheet changes for BP 2.3 (#6291 #6338), we'll be adding SCSS files to src/bp-templates/bp-legacy/css/ to help modernise our CSS workflow. This change adds a SCSS linter to help us maintain our code quality.

An empty src/bp-templates/bp-legacy/css/twentyfifteen.scss file has been added because the linter hangs if you tell it to run in a folder that does not contain any SCSS files. This will be replaced shortly with the real companion stylesheet for twentyfifteen (#6291).

As grunt-scss-lint relies on a ruby gem, the Travis-CI config has been modified to install it before running tests.

Fixes #6351, props netweb.