Use pre-release sass-lint to allow SASS linter exemptions

[maxious]

We have some good exceptions-to-the-rule but where keeping the rule as a whole enabled make sense - we want to use google fonts but we need to look out for other hard coded URLs, we want to use double columns for psuedo-element selectors but when doing very IE specific work they only support single etc.

sass-lint is pretty close to enabling case-by-case exemptions but not quite yet sasstools/sass-lint#677

This PR overrides the dependencies of gulp-sass-lint using npm shrinkwrap to test out that code.

FYI the various exemption syntax types (whole file, selector and children, line only) are detailed in https://github.com/sasstools/sass-lint/blob/c9a3b658aa7b2f06915a108e68c397d24b6451aa/docs/options/toggle-rules-in-src.md

[petronbot]

@maxious I think it would be neater to not include linter exemptions - any times we want to stray from the rules we should find a better way, i.e:

• Include Google Fonts with the JS loader instead of via an @import
• Rethink IE8 support - do we really need multi-columns? If we swap our media queries to mobile-first then our small-screen layout will be a fallback and don't need a clearfix.

What do you think?

[maxious]

That works too. I'll add some bugs for those 2 issues.