Bump govuk_app_config from 2.5.1 to 2.6.0

Bumps [govuk_app_config](https://github.com/alphagov/govuk_app_config) from 2.5.1 to 2.6.0.
- [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@v2.5.1...v2.6.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Gemfile.lock

@@ -112,7 +112,7 @@ GEM
     globalid (0.4.2)
       activesupport (>= 4.2.0)
     govuk_ab_testing (2.4.2)
-    govuk_app_config (2.5.1)
+    govuk_app_config (2.6.0)
       logstasher (>= 1.2.2, < 1.4.0)
       sentry-raven (~> 3.1.1)
       statsd-ruby (~> 1.4.0)