Update govuk_template_mustache from 0.3.4 to 0.3.7

https://github.com/alphagov/govuk_template/compare/v0.3.4...v0.3.7
Merge pull request #111 from alphagov/update-govuk_template-0-3-7

Update govuk_template_mustache from 0.3.4 to 0.3.7