Test Integration tests against Elasticsearch 2.x on Travis

[orangejulius]

This will require some work to figure out how to do it, but I'm sure it can be done.

[orangejulius]

So, one solution to this problem would be to use the standard virtual machine builds on Travis which allow use of sudo, and thus allow us to install whatever ES version we'd like.

On the other hand, they take longer to spin up, slowing down test running. So ideally, we can use something else. The beta "trusty" environment may support a newer version of Elasticsearch, and if that's the case we should definitely use it over setting sudo: true.