Upgrade to ElasticSearch 6

[EnTeQuAk]

• Add ElasticSearch 6 to Travis
• Figure out exact ElasticSearch version

[EnTeQuAk]

Slightly blocked by travis-ci/apt-source-safelist#379 - not a hard block though, we can get around it.

[EnTeQuAk]

Added an upstream patch travis-ci/apt-source-safelist#385

[EnTeQuAk]

@bqbn are there any ops requirements in regards to timing when this has to happen on our end?

[bqbn]

We haven't started any work regarding ES6 upgrade yet. We can probably target this Q1/Q2 2019 from ops perspective.

We'd certainly want to do this after we are done with the UTF8mb4 upgrade (https://bugzilla.mozilla.org/show_bug.cgi?id=1479111), which is slated to happen early Q1.