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/golang/protobuf/proto/equal.go

@@ -246,7 +246,8 @@ func equalExtMap(base reflect.Type, em1, em2 map[int32]Extension) bool {
 			return false
 		}
 
-		m1, m2 := e1.value, e2.value
+		m1 := extensionAsLegacyType(e1.value)
+		m2 := extensionAsLegacyType(e2.value)
 
 		if m1 == nil && m2 == nil {
 			// Both have only encoded form.