Update to use go modules (virtual-kubelet#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

.circleci/config.yml

@@ -3,6 +3,8 @@ jobs:
   validate:
     docker:
       - image: circleci/golang:1.12
+        environment:
+          GO111MODULE: "on"
     working_directory: /go/src/github.com/virtual-kubelet/virtual-kubelet
     steps:
       - checkout
@@ -11,17 +13,19 @@ jobs:
           command: V=1 CI=1 make vet
       - run:
           name: Install linters
-          command: curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | bash -s v1.10.2 && mv ./bin/* /go/bin/
+          command: curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | bash -s v1.15.0 && mv ./bin/* /go/bin/
       - run:
           name: Lint
           command: golangci-lint run --new-from-rev "HEAD~$(git rev-list master.. --count)" ./...
       - run:
           name: Dependencies
-          command: scripts/validate/dep.sh
+          command: scripts/validate/gomod.sh
 
   test:
     docker:
       - image: circleci/golang:1.12
+        environment:
+          GO111MODULE: "on"
     working_directory: /go/src/github.com/virtual-kubelet/virtual-kubelet
     steps:
       - checkout
@@ -58,6 +62,7 @@ jobs:
       MINIKUBE_WANTUPDATENOTIFICATION: false
       MINIKUBE_WANTREPORTERRORPROMPT: false
       SKAFFOLD_VERSION: v0.18.0
+      GO111MODULE: "on"
     steps:
       - checkout
       - run: