Update govuk_frontend_toolkit to 2.0.1

# 2.0.1

- Fix new grid helpers to ensure content using %site-width-container is
centred in IE.

# 2.0.0

- Add support for selection-button events to be document-level
https://github.com/alphagov/govuk_frontend_toolkit/pull/139

- The `GOVUK.selectionButtons` interface is now deprecated. Details are
in the
[README](https://github.com/alphagov/govuk_frontend_toolkit#deprecated-f
unctionality).
Update SelectionButtons to new interface