build: drop v6.x support

[mmarchini]

Node.js v6.x is EOL. Keeping it around makes it more dificult to support new versions, so dropping support seems reasonable. This is semver-major.

[codecov-io]

Codecov Report

Merging #292 into master will decrease coverage by 1.93%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
- Coverage   81.79%   79.85%   -1.94%     
==========================================
  Files          33       33              
  Lines        4229     4229              
==========================================
- Hits         3459     3377      -82     
- Misses        770      852      +82

Impacted Files	Coverage Δ
src/error.h	85.71% <0%> (-14.29%)	:arrow_down:
test/plugin/workqueue-test.js	88.88% <0%> (-11.12%)	:arrow_down:
src/printer.cc	79.51% <0%> (-6.2%)	:arrow_down:
src/llv8-constants.cc	83.49% <0%> (-5.83%)	:arrow_down:
src/node-constants.cc	88.88% <0%> (-3.18%)	:arrow_down:
src/llv8-inl.h	92.67% <0%> (-2.93%)	:arrow_down:
src/node.cc	87.8% <0%> (-2.44%)	:arrow_down:
src/llnode.cc	74.2% <0%> (-0.8%)	:arrow_down:
src/llv8.cc	75.46% <0%> (-0.62%)	:arrow_down:
test/common.js	87% <0%> (-0.57%)	:arrow_down:

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[mmarchini]

cc @nodejs/llnode @nodejs/diagnostics @nodejs/post-mortem for visibility

[mmarchini]

Landed in e1a74b0