Update Jenkinsfile
Update Jenkinsfile

This installs yarn dependencies as they are required for linting. It
removes the tie to PostgreSQL 9.3 as this app is now running on 9.6. It
also removes the `rubyLintDiff` argument as this no longer does
anything.

alphagov/service-manual-publisher,alphagov/service-manual-publisher,alphagov/service-manual-publisher