Beats version 6.4.0 support

[at-k]

Elastic beats is now version 6.4.0(release notes). This PR includes following updates to support 6.4.0

• follow up the update at json.New function made at this PR.
• set default beats version 6.4.0 at Makefile
• modify ignored field to skip installing beats into vendor/* in order to avoid to use it for build

[codecov-io]

Codecov Report

Merging #47 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #47   +/-   ##
=======================================
  Coverage   64.44%   64.44%           
=======================================
  Files           4        4           
  Lines         135      135           
=======================================
  Hits           87       87           
  Misses         42       42           
  Partials        6        6

Impacted Files	Coverage Δ
streams/client.go	69.79% <0%> (ø)	⬆️

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[s12v]

thanks, @at-k