Added pluginManagement section.
Added pluginManagement section.

Change-Id: I9ce993f5b6cddf90314920cc1ddf8b55985dd018

GoogleCloudPlatform/cloud-pubsub-samples-java,googlearchive/cloud-pubsub-samples-java,GoogleCloudPlatform/cloud-pubsub-samples-java,GoogleCloudPlatform/cloud-pubsub-samples-java,googlearchive/cloud-pubsub-samples-java,googlearchive/cloud-pubsub-samples-java