SCHEMA-578. protobuf library dependency conflict between Kiji and HBase/Hadoop

 - Downgrade protobuf dependency from 2.5.0 to 2.4.0a to match HBase/Hadoop version.
 - Make protobuf dependency provided.

issue: https://jira.kiji.org/browse/SCHEMA-578
review: https://review.kiji.org/r/1197