Disable go mod once in travis

[shu-mutou]

We need to investigate to use go mod in travis.
For now, change GO111MODULE to off in travis.

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[shu-mutou]

npm ci seems to update go.mod file to use latest. Also, prometheus/client-golang was bumped latest to 1.0.0 few days ago, then prometheus.Handler() removed.

[maciaszczykm]

Hmm, @eloyekunle can you help us there?

[codecov]

Codecov Report

Merging #3935 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3935   +/-   ##
=======================================
  Coverage   46.63%   46.63%           
=======================================
  Files         176      176           
  Lines        8133     8133           
  Branches       71       71           
=======================================
  Hits         3793     3793           
  Misses       4106     4106           
  Partials      234      234

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[shu-mutou]

I updated this PR as disabling go mod in travis.
Also, I posted Issue #3936. @eloyekunle Could you investigate this issue?

[maciaszczykm]

cc @floreks AFAIK you enabled Go modules. We need them, arent we?

[shu-mutou]

Basically, I agree with using go mod, but we need to investigate it more. 😉

[eloyekunle]

I will look into it.

[eloyekunle]

go get for golangci-lint in the postinstall script was updating some packages. In #3934, we download golangci-lint via curl instead.

[shu-mutou]

@eloyekunle Thank you for your help! I understood we should not use go get to install tools.