test: add node 8

[fengmk2]

No description provided.

[mention-bot]

@fengmk2, thanks for your PR! By analyzing the history of the files in this pull request, we identified @DylanPiercey and @dead-horse to be potential reviewers.

[codecov]

Codecov Report

Merging #49 into master will decrease coverage by 2.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   92.91%   90.65%   -2.27%     
==========================================
  Files           4        4              
  Lines         254      246       -8     
==========================================
- Hits          236      223      -13     
- Misses         18       23       +5

Impacted Files	Coverage Δ
lib/_http_agent.js	89.24% <0%> (-2.99%)	⬇️
lib/agent.js	93.18% <0%> (-0.16%)	⬇️

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