Ignore Perf Platform's "backdrop" during syncs
Ignore Perf Platform's "backdrop" during syncs

Since PP migrated to GOV.UK infrastucture the sync to replicate
data is taking a lot longer, especially the Mongo 'backdrop' DB.

Add it to the common ignore list, so GOV.UK developers can sync
the data they need more quickly.

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