Use a Github branch of govuk_app_config for testing

We're instrumenting Puma in that branch.

Author: sihugh

Gemfile

@@ -9,7 +9,7 @@ gem "dalli"
 gem "dartsass-rails"
 gem "gds-api-adapters"
 gem "govuk_ab_testing"
-gem "govuk_app_config"
+gem "govuk_app_config", git: "https://www.github.com/alphagov/govuk_app_config", branch: "rails-metrics"
 gem "govuk_personalisation"
 gem "govuk_publishing_components"
 gem "govuk_web_banners"