Add node 8 to test matrix

[danez]

No description provided.

[codecov]

Codecov Report

Merging #467 into master will increase coverage by 1.72%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #467      +/-   ##
=========================================
+ Coverage   78.77%   80.5%   +1.72%     
=========================================
  Files           6       6              
  Lines         179     200      +21     
  Branches       46      56      +10     
=========================================
+ Hits          141     161      +20     
  Misses         18      18              
- Partials       20      21       +1

Impacted Files	Coverage Δ
src/resolve-rc.js	90% <0%> (-1.67%)	⬇️
src/index.js	85.71% <0%> (+2.01%)	⬆️

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