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/base/api_base.proto

@@ -0,0 +1,33 @@
+// Built-in base types for API calls. Primarily useful as return types.
+
+syntax = "proto2";
+option go_package = "base";
+
+package appengine.base;
+
+message StringProto {
+  required string value = 1;
+}
+
+message Integer32Proto {
+  required int32 value = 1;
+}
+
+message Integer64Proto {
+  required int64 value = 1;
+}
+
+message BoolProto {
+  required bool value = 1;
+}
+
+message DoubleProto {
+  required double value = 1;
+}
+
+message BytesProto {
+  required bytes value = 1 [ctype=CORD];
+}
+
+message VoidProto {
+}