Need to use latest version of lighthouse

[ankitjain0105]

Summary

Need to update the package.json file to use the latest version of lighthouse

Expected Behaviour

Lighthouse version should be latest

Current Behaviour

a11y-tests-web uses lighthouse v2.7.0 - see here https://github.com/bbc/a11y-tests-web/blob/master/package.json#L27

Possible Solution

We should update to use the latest version of lighthouse

[andymsuk]

Thanks @ankitjain0105.

We're currently 3 major versions behind so it's likely to need quite a big code change in addition to the version bumping.

If anyone wants to take a look and a submit a PR, that would be great! 😃