Bump front end toolkit version

Update version for the addition of new grid helpers

The npm will then be updated and I can update the GOV.UK elements grid examples.
I know we're not doing it in the rest of the the CHANGELOG, so feel free to ignore, but it would be nice to use the markdown code highlighting, eg:

- The old grid mixins, `@outer-block` and `@inner-block` are now deprecated

Makes it a bit clearer when an interface-like thing has changed.