Version bump to 3.1.0

- Fix: outdent to add right margin rather than only left
- Fix: add missing semi-colons in JavaScript files
- Fix: use box-sizing mixin in column mixin to support more browsers
- Add: ability to specify float direction on column mixin
- Add: Sass-lint tests

Full changes: https://github.com/alphagov/govuk_frontend_toolkit/compare/dce0806...0c99f78