[ci] update Elasticsearch versions in CI and docs

[jameslamb]

it's been a few months since we updated versions we test against (#209 ). This PR does that again.

• 6.8.8 --> 6.8.11
• +7.7.1, +7.8.1 (previous latest was 7.6.2)

This is the page I use to find what versions have been released: https://www.elastic.co/downloads/past-releases#elasticsearch

[codecov-commenter]

Codecov Report

Merging #213 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #213   +/-   ##
=======================================
  Coverage   93.13%   93.13%           
=======================================
  Files           8        8           
  Lines         597      597           
=======================================
  Hits          556      556           
  Misses         41       41           

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[austin3dickey]

good times!! Looks like the Travis build didn't kick off?

[jameslamb]

I saw this same Travis issue on LightGBM too (microsoft/LightGBM#3311 (comment)) :/

I contacted Travis support. I had a similar issue in April...I think the way they're handling the rollout of travis-ci.com (the replacement to travis-ci.org) has caused some problems.

[jameslamb]

this build just passed, it's just not reporting back :/

https://travis-ci.com/github/jameslamb/uptasticsearch/builds/180065938

I'm gonna try closing and re-opening this PR

[jameslamb]

Hey I got the details link to pop up! For reference, I had to go to https://travis-ci.org/github/uptake/uptasticsearch and restart the build from there. That seemed to help when restarting here on GitHub (by pushing an empty commit and by closing and re-opening the PR) did not