Update scss-lint.yml

This is part of an update to support Sass 3.4.

The latest version (0.30.0) of scss-lint doesn't work with Sass 3.3, so while I'm updating modules to work with Sass 3.4, this leaves us time to think about linting rules.

I've mainly updated the rules to avoid bad surprises, and also made a few changes:
- **severity:** error/warning: not all linting issues are equal in severity.
    - error: really big problems that jeopardize the "grep-ability" of the codebase we think should break the build
    - warning: issues that can be ignored (at the author's discretion), and we think should not prevent people from deploying
- **indentation:** we can now enforce tab indentation with the new version of scss-lint! Not sure all authors will like that, though.
- Selector, mixin, function, variable naming conventions: I set the linter to only allow chars covered in the "\w" regex range. This will avoid having any crazy unicode chars in our CSS.

What do you think about the severity settings?

This is a space for discussion, please share your thoughts now and we'll roll this out when everybody is comfortable with it. But remember that we can always tweak settings later if we find something doesn't make sense in this configuration.
Slightly more consistent way of documenting this would be a line to say what it does and then a second tosay why we're turning it off.