Replace intern with ava & drop Node < 8

[malept]

Modernize sumchecker (Node 6 is EOL on 2019-04-30). A future PR will convert the main code to use async/await syntax.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@7fd8e38). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #10   +/-   ##
=======================================
  Coverage          ?   100%           
=======================================
  Files             ?      1           
  Lines             ?     79           
  Branches          ?      0           
=======================================
  Hits              ?     79           
  Misses            ?      0           
  Partials          ?      0

Impacted Files	Coverage Δ
index.js	100% <100%> (ø)

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