Spark 2.3 Support

[rbrush]

Spark 2.3 is out and gaining traction, so we should support it. Ideally we should be able to produce a build that supports both Spark 2.2 and 2.3.

[rbrush]

After some experiments, the same Bunsen code will work for Spark 2.2 and 2.3, but we will need to recompile for the newer version. For the time being we will limit the scope fo this change to getting common code that will work to simplify a future state if we need to deploy versions for 2.2 and 2.3.

[rbrush]

Closing this as master can now be compiled and run against either 2.2 or 2.3. We can create separate releases for these versions in the future.