Remove ChefDK’s Omnitruck JSON

Now that https://github.com/opscode-cookbooks/opscode-ci/pull/149 has
been merged we no longer need to maintain build -> test platform
mappings in multiple locations. The pipeline’s build to test matrix is
now the single source of truth. Once the Chef pipelines have been fully
migrated over to the Manhattan cluster in new CI we can also remove:
`jenkins/release.rb`.
Update to the latest Omnibus