Pin patch version of govuk-elements-sass
Pin patch version of govuk-elements-sass

Version 3.1.3 changed heading classes to display block - https://github.com/alphagov/govuk_elements/pull/552
This is a breaking change for us since we are using the heading classes
to make font bold - 3.1.3 adds line breaks in places where we don't want
them and causes some functional tests to fail.

Since we will be replacing govuk-elements with the new design system,
this commit pins the version of govuk-elements instead of updating all
the code to work with the lastest version.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin