Add conditional test run on elastic search

[lakhina]

fixes #48

[TimMoore]

God job working through this, @lakhina and @yg-apaza!

Now that we're using sudo, we could also install using the deb package as described in https://docs.travis-ci.com/user/database-setup/#Installing-specific-versions-of-ElasticSearch

I think we can merge this as is, though, and save that change for a future improvement.

I'll leave this open for @ignasi35 to give a second opinion before merging.

[ignasi35]

This looks great!

I will merge as is and create 2 issues:

• track potential changes on installing ES via .deb files. IIUC there was some work on that front already but it happened before using sudo: required. See Use .deb installers in Travis #63
• restore sbt +test. I'm not sure we need to run the tests for multiple scala versions but want to have the reasons of that change tracked. See Run tests on multiple scala versions #62