build(deps): bump govuk_app_config from 2.5.0 to 2.5.1

Bumps [govuk_app_config](https://github.com/alphagov/govuk_app_config) from 2.5.0 to 2.5.1.
- [Release notes](https://github.com/alphagov/govuk_app_config/releases)
- [Changelog](https://github.com/alphagov/govuk_app_config/blob/master/CHANGELOG.md)
- [Commits](alphagov/govuk_app_config@v2.5.0...v2.5.1)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -133,7 +133,7 @@ GEM
       jquery-rails (~> 4.3.1)
       plek (>= 2.1.0)
       rails (>= 3.2.0)
-    govuk_app_config (2.5.0)
+    govuk_app_config (2.5.1)
       logstasher (>= 1.2.2, < 1.4.0)
       sentry-raven (~> 3.1.1)
       statsd-ruby (~> 1.4.0)