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.

vendor/github.com/google/gofuzz/.travis.yml

@@ -0,0 +1,13 @@
+language: go
+
+go:
+  - 1.4
+  - 1.3
+  - 1.2
+  - tip
+
+install:
+  - if ! go get code.google.com/p/go.tools/cmd/cover; then go get golang.org/x/tools/cmd/cover; fi
+
+script:
+  - go test -cover