fix: drop support for node.js 4.x

[JustinBeckwith]

BREAKING CHANGE: Node.js 4.x is no longer supported.

[codecov-io]

Codecov Report

Merging #401 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #401      +/-   ##
=========================================
- Coverage   94.86%   94.8%   -0.06%     
=========================================
  Files          14      14              
  Lines         974     944      -30     
  Branches      207     219      +12     
=========================================
- Hits          924     895      -29     
+ Misses         50      49       -1

Impacted Files	Coverage Δ
src/auth/iam.ts	75% <0%> (-15%)	⬇️
src/auth/googleauth.ts	94.46% <0%> (-0.54%)	⬇️
src/auth/computeclient.ts	95.34% <0%> (-0.11%)	⬇️
src/transporters.ts	95.91% <0%> (-0.09%)	⬇️
src/auth/oauth2client.ts	94.4% <0%> (-0.08%)	⬇️
src/auth/jwtaccess.ts	98.11% <0%> (-0.07%)	⬇️
src/auth/authclient.ts	100% <0%> (ø)	⬆️
src/auth/loginticket.ts	100% <0%> (ø)	⬆️
src/pemverifier.ts	100% <0%> (ø)	⬆️
src/auth/jwtclient.ts	94.84% <0%> (+0.78%)	⬆️
... and 2 more

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[ofrobots]

Commit message should include 'BREAKING CHANGE' as this is a major. This can be edited at landing time.

LGTM.