Update .circleci script with go1.11.1 and better coverage

[mstoykov]

No description provided.

[codecov-io]

Codecov Report

Merging #813 into master will increase coverage by 6.34%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #813      +/-   ##
==========================================
+ Coverage   63.52%   69.86%   +6.34%     
==========================================
  Files         104      111       +7     
  Lines        8529     8679     +150     
==========================================
+ Hits         5418     6064     +646     
+ Misses       2741     2215     -526     
- Partials      370      400      +30

Impacted Files	Coverage Δ
js/lib/rice-box.go	100% <0%> (ø)
lib/testutils/httpmultibin.go	95.12% <0%> (ø)
api/v1/client/client.go	0% <0%> (ø)
api/common/context.go	100% <0%> (ø)
api/v1/client/status.go	0% <0%> (ø)
api/v1/client/metrics.go	0% <0%> (ø)
js/lib/lib.go	100% <0%> (ø)
js/modules/k6/html/html.go	85.08% <0%> (+0.8%)	⬆️
js/modules/k6/http/http.go	78.18% <0%> (+3.63%)	⬆️
lib/netext/tracer.go	97.89% <0%> (+4.21%)	⬆️
... and 22 more

---

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[na--]

btw, it might also be useful to put something like export GOMAXPROCS=N in the test stages, where N is 2 or 4 or something sensible like that. Take a look at this build, the CircleCI machines have quite a few cores, but only 2 of those are accessible to the docker container that the build is running in. And I think that by default, go schedules tests based on the number of cores

[na--]

Ah, the build error is because the new gihub PR suggestion seems to be one-line only, or at least I don't know how to make multi-line suggestions and I can't find the docs about it...

[na--]

Ah... FAIL github.com/loadimpact/k6/js 292.870s 🤕
Merge master in the branch?

[na--]

LGTM