CI: Workaround OOM NYC issue

[medikoo]

We started to observe OOM errors when doing coverage check on Node.js v10: https://travis-ci.com/github/serverless/enterprise-plugin/jobs/305196115

Issue is known on NYC side: istanbuljs/nyc#1263 and they have fix coming: istanbuljs/nyc#1293

I was not able to reproduce this when running on Node.js v13 and v12, hence I've updated CI to run coverage on v13 (which we also didn't test yet)

[codecov]

Codecov Report

Merging #394 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #394   +/-   ##
=======================================
  Coverage   54.39%   54.39%           
=======================================
  Files         105      105           
  Lines        2960     2960           
=======================================
  Hits         1610     1610           
  Misses       1350     1350           

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