Upgrade govuk_template_mustache to 0.9.1

https://github.com/alphagov/govuk_template/compare/v0.8.1...v0.9.1
Upgrade frontend toolkit from 1.4.0 to 2.0.0

https://github.com/alphagov/govuk_frontend_toolkit/blob/03380d5577ce65d6680220294ca7861b58fa15a9/CHANGELOG.md
Merge pull request #831 from alphagov/upgrade-govuk-dependencies

Upgrade GOV.UK dependencies