Upgrade java-driver to 2.0.7 and downgrade Guava to 14.0.1.
Spark runs with Guava 14. By not requiring Guava 15 in the java driver anymore we improve compatibility and now spark connector is compatible with versions 14, 15, 16 and probably (not tested) 17 and 18. Guava 14 is now also packaged in sbt assembly task.  Unfortunately, cassandra server used in the integration tests requires Guava 15, so beware tests are still using Guava 15.
Fixes #366.