Bump the govuk_admin_template gem to 1.1.5

- This new version fixes our issue with assets not loading on
  postgres-master (see the changelog for 1.1.4 - https://github.com/alphagov/govuk_admin_template/commit/fc76a59a1a7ac1a636a56d7bc83b8a9771eda657),
  and avoids potentially breaking loading of rake tasks.