Switch imports to sigs.k8s.io/krew

[soltysh]

/assign @ahmetb

[corneliusweig]

@soltysh There are quite a few references to the github.com/kubernetes-sigs/krew repo left.
I think this is also the reason why travis fails, because it still says

go_import_path: github.com/kubernetes-sigs/krew

[codecov-io]

Codecov Report

Merging #181 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #181   +/-   ##
=======================================
  Coverage   50.91%   50.91%           
=======================================
  Files          13       13           
  Lines         711      711           
=======================================
  Hits          362      362           
  Misses        297      297           
  Partials       52       52

Impacted Files	Coverage Δ
pkg/environment/environment.go	77.27% <ø> (ø)	⬆️
cmd/validate-krew-manifest/main.go	37.63% <ø> (ø)	⬆️
pkg/installation/util.go	61.25% <ø> (ø)	⬆️
pkg/installation/move.go	32.55% <ø> (ø)	⬆️
pkg/installation/install.go	35.16% <ø> (ø)	⬆️
pkg/installation/upgrade.go	0% <ø> (ø)	⬆️
pkg/index/indexscanner/scanner.go	67.34% <ø> (ø)	⬆️

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[ahmetb]

/lgtm
/approve

[k8s-ci-robot]

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