Trigger downstream project builds from Jenkinsfile
Trigger downstream project builds from Jenkinsfile

When the schemas are updated, we need to ensure that projects which
depend on those schema still build correctly.

This commit adds the first dependent project (collections-publisher).
The rest should be added once they have been migrated to Jenkins 2 and
deployed to production.

alphagov/govuk-content-schemas,alphagov/govuk-content-schemas