Update for Travis CI to test against spark 2.0

[boazmohar]

Address #359

[jwittenbach]

Well look at that -- awesome! 👍

[freeman-lab]

Well that was straightforward 🎉 thanks @boazmohar ! Can you add a note to the README as part of this PR saying that it works with Spark 2.0.0?

[boazmohar]

Yep, was very easy. I guess the parts that changed a lot in 2.0 are DataFrame and Dataset not RDD's.

[jwittenbach]

I've been thinking that it would be test whether there might be any speed-up in switching things to DataFrames -- though I haven't had any time to play with it.

[freeman-lab]

Yeah agreed @jwittenbach, we should definitely look into that, I think the Dataset API may be what we want to look at. But in the meantime let's merge this in, good to make clear it's working with the latest version.