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.
Merge pull request #380 from datastax/wip-366-upgrade-java-driver

Wip 366 upgrade java driver
Calculate implicit TypeTags for leaf types in TypeConverter companion object constructor, rather than in targetTypeTag method. Therefore they are initialized all exactly once and there is no risk of race conditions, and no need to acquire a lock whenever they are referenced. Synchronize many methods which construct type tags on demand. Fixes #364.