Add the scss-lint dependency
Add the scss-lint dependency

The plugin `gulp-scss-lint` requires `scss-lint` to be installed. This is a ruby dependency,
so adding the Gemfile required to perform this task.

scss-lint offers a lot of features, and linting options:
https://github.com/causes/scss-lint/blob/master/lib/scss_lint/linter/README.md

theorchard/linters,theorchard/linters