Add ruby and sass linting

[fofr]

The template has no linting.

• source directory needs sass linting
• build_tools, Gemfiles and lib should be ruby linted

I've started in this branch:
https://github.com/alphagov/govuk_template/tree/lint

• Linting requires a sass gem upgrade which needs a sprockets bump
• The fonts.css file can't easily be excluded from the linter

[36degrees]

Following the launch of the GOV.UK Design System, GOV.UK Template will now only get major bug fixes and security patches, so I'm going to close this.