Add confluent maven reposetory to kstreams-commons

solves #253 (closed).

The maven repository is mising in the KafkaStreams-Common Repositories. This can result in problehms when new imporitng all project with a clean cache into an IDE. For comparison, the same repository is used in the Flink-Common project:

maven { url 'https://packages.confluent.io/maven/' }