replace "dep" with "go mod"

A simple "go mod init && go mod tidy && go mod vendor" was enough and
mostly seems to have picked the same revisions of the dependencies as
before. Notable exceptions are klog and gogo/protobuf.

vendor/sigs.k8s.io/yaml/.travis.yml

@@ -0,0 +1,14 @@
+language: go
+dist: xenial
+go:
+  - 1.9.x
+  - 1.10.x
+  - 1.11.x
+script:
+  - go get -t -v ./...
+  - diff -u <(echo -n) <(gofmt -d .)
+  - diff -u <(echo -n) <(golint $(go list -e ./...) | grep -v YAMLToJSON)
+  - go tool vet .
+  - go test -v -race ./...
+install:
+  - go get golang.org/x/lint/golint