Drop Node 4 support

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes are appropriately documented (if applicable).
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Fixes #815.

[codecov]

Codecov Report

Merging #900 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #900   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines         643    641    -2     
=====================================
- Hits          643    641    -2

Impacted Files	Coverage Δ
mac.js	100% <100%> (ø)	⬆️
ignore.js	100% <100%> (ø)	⬆️
infer.js	100% <100%> (ø)	⬆️
platform.js	100% <100%> (ø)	⬆️
win32.js	100% <100%> (ø)	⬆️
common.js	100% <100%> (ø)	⬆️
targets.js	100% <100%> (ø)	⬆️
prune.js	100% <100%> (ø)	⬆️

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