chore: add back node v6 support

[nfischer]

No change to logic. This adds back node v6 support to partially revert
#186. I'm hoping that v6 support is
easier now that we rely on GitHub actions for codecov. The motivation
for this change is so that we can make 0.3.x patch releases if
necessary.

[codecov-commenter]

Codecov Report

Merging #196 (c2d3693) into master (92d8c42) will not change coverage.
The diff coverage is n/a.

❗ Current head c2d3693 differs from pull request most recent head 635e287. Consider uploading reports for the commit 635e287 to get more accurate results

@@            Coverage Diff            @@
##            master      #196   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            5         5           
  Lines          114       114           
=========================================
  Hits           114       114           

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