Consume MRI dependency from the dep-server

[sophiewigmore]

Per paketo-buildpacks/dep-server#45, we should start consuming the MRI dependency from the dep-server (https://api.deps.paketo.io/v1/dependency?name=ruby) instead of the https://buildpacks.cloudfoundry.org/dependencies/ruby/... location we currently get the dependencies from.

This will make the dependency publishing/consumption process more transparent than the process we use for the dependencies available via the dependency-builds pipeline.

We have already done this switch-over in the Node Engine and Yarn Buildpacks. The outline of what this work will entail can be found in the dep-server issue linked at the top.