build: stop testing on Node 6

[ofrobots]

NOTE: this targets the next branch.

[codecov-io]

Codecov Report

Merging #256 into next will decrease coverage by 0.24%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             next     #256      +/-   ##
==========================================
- Coverage   98.54%   98.29%   -0.25%     
==========================================
  Files          12       12              
  Lines         823      823              
  Branches       69       69              
==========================================
- Hits          811      809       -2     
- Misses         12       14       +2

Impacted Files	Coverage Δ
test/test-kitchen.ts	97.8% <0%> (-2.2%)	⬇️

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[JustinBeckwith]

Just leaving a nah here so we don't merge this until april though.

[JustinBeckwith]

I am terrible at reading.