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/google.golang.org/appengine/internal/remote_api/remote_api.proto

@@ -0,0 +1,44 @@
+syntax = "proto2";
+option go_package = "remote_api";
+
+package remote_api;
+
+message Request {
+  required string service_name = 2;
+  required string method = 3;
+  required bytes request = 4;
+  optional string request_id = 5;
+}
+
+message ApplicationError {
+  required int32 code = 1;
+  required string detail = 2;
+}
+
+message RpcError {
+  enum ErrorCode {
+    UNKNOWN = 0;
+    CALL_NOT_FOUND = 1;
+    PARSE_ERROR = 2;
+    SECURITY_VIOLATION = 3;
+    OVER_QUOTA = 4;
+    REQUEST_TOO_LARGE = 5;
+    CAPABILITY_DISABLED = 6;
+    FEATURE_DISABLED = 7;
+    BAD_REQUEST = 8;
+    RESPONSE_TOO_LARGE = 9;
+    CANCELLED = 10;
+    REPLAY_ERROR = 11;
+    DEADLINE_EXCEEDED = 12;
+  }
+  required int32 code = 1;
+  optional string detail = 2;
+}
+
+message Response {
+  optional bytes response = 1;
+  optional bytes exception = 2;
+  optional ApplicationError application_error = 3;
+  optional bytes java_exception = 4;
+  optional RpcError rpc_error = 5;
+}