Bump version to 3.0.1

This publishes the fixes included in https://github.com/alphagov/govuk_frontend_toolkit_npm/pull/8, which is where I used the files array in package.json to only specify `README.md`. This whitelists files, so you end up with a toolkit that only included a README. Which is not useful.

This doesn't include any changes to the toolkit, but npmjs.org requires the version to be changed to publish the package.