all: add go1.13rc1 to list of supported Go versions

Change-Id: I2ba52055bcb0e434b66dbcd4763afd2fce5b4264
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/191179
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/cmd/protoc-gen-go-grpc/testdata/go.mod b/cmd/protoc-gen-go-grpc/testdata/go.mod
index acca211..fb7cb8b 100644
--- a/cmd/protoc-gen-go-grpc/testdata/go.mod
+++ b/cmd/protoc-gen-go-grpc/testdata/go.mod
@@ -1,5 +1,7 @@
 module google.golang.org/protobuf/cmd/protoc-gen-go-grpc/testdata
 
+go 1.9
+
 require (
 	github.com/golang/protobuf v1.2.1-0.20190820213554-ae1d65bc5435
 	google.golang.org/grpc v1.19.0
diff --git a/cmd/protoc-gen-go/testdata/go.mod b/cmd/protoc-gen-go/testdata/go.mod
index 0eb1883..a72c848 100644
--- a/cmd/protoc-gen-go/testdata/go.mod
+++ b/cmd/protoc-gen-go/testdata/go.mod
@@ -1,5 +1,7 @@
 module google.golang.org/protobuf/cmd/protoc-gen-go/testdata
 
+go 1.9
+
 require (
 	github.com/golang/protobuf v1.2.1-0.20190820213554-ae1d65bc5435
 	google.golang.org/protobuf v1.0.0
diff --git a/integration_test.go b/integration_test.go
index c91b227..35fbfef 100644
--- a/integration_test.go
+++ b/integration_test.go
@@ -34,7 +34,7 @@
 	buildRelease = flag.Bool("buildRelease", false, "build release binaries")
 
 	protobufVersion = "3.9.1"
-	golangVersions  = []string{"1.9.7", "1.10.8", "1.11.13", "1.12.9"}
+	golangVersions  = []string{"1.9.7", "1.10.8", "1.11.13", "1.12.9", "1.13rc1"}
 	golangLatest    = golangVersions[len(golangVersions)-1]
 
 	// purgeTimeout determines the maximum age of unused sub-directories.