Upgraded cassandra to 2.0.6.

 - Main motivation is the bug CASSANDRA-6687
 - Upgraded guava to match cassandra 2.0.6 version, to be considered if
   this dependency should not be removed from casssandra-unit's pom.xml
   as it is resolved either way via cassandra, or if the cassandra guava
   dependency should not be excluded

 - 6687 url: https://issues.apache.org/jira/browse/CASSANDRA-6687