Add support for Elasticsearch 6

[jameslamb]

This PR adds support for Elasticsearch 6.x!!! This has been long overdue (Elasticsearch 6.0.0 came out in 7 months ago)...sorry for the delay.

The most relevant change (for this package) in ES 6.x was the strict enforcement of content type on HTTP requests. TL;DR previously ES just figured it out automagically for you, which made debugging kind of hard and was also a security risk. ES6 is now more verbose and will break more often when you don't tell the service what content-type is being exchanged in requests.

For more, see this blog post from Elastic's engineering team on the issue: https://www.elastic.co/blog/strict-content-type-checking-for-elasticsearch-rest-requests.

addresses #43

[codecov-io]

Codecov Report

Merging #64 into master will decrease coverage by 2.59%.
The diff coverage is 30.3%.

@@            Coverage Diff            @@
##           master      #64     +/-   ##
=========================================
- Coverage   72.11%   69.52%   -2.6%     
=========================================
  Files           3        3             
  Lines         538      502     -36     
=========================================
- Hits          388      349     -39     
- Misses        150      153      +3

Impacted Files	Coverage Δ
R/elasticsearch_eda_funs.R	41.86% <19.04%> (-1.52%)	⬇️
R/elasticsearch_parsers.R	74.93% <50%> (-2.18%)	⬇️

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[jameslamb]

ayyy this worked!

[austin3dickey]

Nice

[jameslamb]

@austin3dickey thanks for reviewing