Update to use go modules (#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.

Makefile.e2e

@@ -36,7 +36,7 @@ e2e: NODE_NAME := vkubelet-mock-0
 e2e: export VK_BUILD_TAGS += mock_provider
 e2e: e2e.clean bin/e2e/virtual-kubelet skaffold/run
 	@echo Running tests...
-	cd $(PWD)/internal/test/e2e && go test -v -timeout 5m -tags e2e ./... \
+	cd $(PWD)/internal/test/e2e && go test -mod=vendor -v -timeout 5m -tags e2e ./... \
 		-kubeconfig=$(KUBECONFIG) \
 		-namespace=$(NAMESPACE) \
 		-node-name=$(NODE_NAME) \