Add publishing-api as a dependency for CPM
Add publishing-api as a dependency for CPM

Content Performance Manager is using publishing-api to retrieve the
list of the Content Items that are published in order to calculate
metrics.

We have decided to go with the Publishing API as some metrics that we
have planned to include require attributes that only exist in Publishing 
API.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet