Move to go modules (vgo)

[ahmetb]

Fixes #234.
/assign @corneliusweig

[codecov-io]

Codecov Report

Merging #235 into master will increase coverage by 0.17%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #235      +/-   ##
==========================================
+ Coverage    53.6%   53.78%   +0.17%     
==========================================
  Files          16       14       -2     
  Lines         735      701      -34     
==========================================
- Hits          394      377      -17     
+ Misses        289      273      -16     
+ Partials       52       51       -1

Impacted Files	Coverage Δ
pkg/environment/environment.go	77.27% <0%> (-1.9%)	⬇️
pkg/installation/upgrade.go	0% <0%> (ø)	⬆️
pkg/receipt/receipt.go
pkg/info/info.go
pkg/installation/install.go	36.36% <0%> (+4.36%)	⬆️

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[corneliusweig]

This PR is too big, I can't add inline comments 😆

• .travis.yml/GOPROXY: I can't find that in the travis guide for building go projects (https://docs.travis-ci.com/user/languages/go/). Why do you set that here?
• PR Save install receipt #195 added github.com/google/go-cmp v0.3.0. That's what causes the conflicts.
• The gox install hack really looks nasty, as it is even inlined in .travis.yml. If you don't see the chance we can get rid of that soonish, I'd prefer a proper script there.

[ahmetb]

• .travis.yml/GOPROXY: I can't find that in the travis guide for building go projects (docs.travis-ci.com/user/languages/go). Why do you set that here?

Anytime you fetch go modules, using GOPPROXY is much faster. See http://proxy.golang.org/. This speeds up the test suite. e.g.

time go mod download => 27s
go clean -modcache -cache
time env GOPROXY=https://proxy.golang.org go mod download => 4s.

That's what causes the conflicts.

Addressed merge conflicts.

• The gox install hack really looks nasty, as it is even inlined in .travis.yml. If you don't see the chance we can get rid of that soonish, I'd prefer a proper script there.

Solved this by introducing a hack/install-gox.sh.

[corneliusweig]

/lgtm
/approve

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