Update govuk_template_mustache to v0.3.4

Diff: https://github.com/alphagov/govuk_template/compare/v0.2.2...v0.3.4

The GOV.UK header has changed on preview, indicating that we should probably
be keeping up.
Merge pull request #103 from alphagov/update-govuk-template-0-3-4

Update govuk_template_mustache to v0.3.4