Upgrade RocksDB dependency to legally acceptable version

RocksDB 5.5.5+ seems to be legally acceptable. For more info see

• https://issues.apache.org/jira/browse/LEGAL-303 and
• https://www.apache.org/legal/resolved.html#category-x

Even latest trunk of Apache Kafka depends on older RocksDB https://github.com/apache/kafka/blob/trunk/gradle/dependencies.gradle#L67

If I'm not mistaken, this makes all current Apache Kafka 0.10+ releases not legally acceptable Apache products.

Please consider upgrading the dependency. If possible please include the change in Apache Kafka 1.0.0 release, if not also in patch releases of older still supported 0.x Apache Kafka branches.