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/grpc-ecosystem/grpc-gateway/internal/stream_chunk.proto

@@ -0,0 +1,15 @@
+syntax = "proto3";
+package grpc.gateway.runtime;
+option go_package = "internal";
+
+import "google/protobuf/any.proto";
+
+// StreamError is a response type which is returned when
+// streaming rpc returns an error.
+message StreamError {
+	int32 grpc_code = 1;
+	int32 http_code = 2;
+	string message = 3;
+	string http_status = 4;
+	repeated google.protobuf.Any details = 5;
+}