all: update protobuf toolchain dependency

This updates the dependency on the protobuf toolchain to an unreleased
version that has the go_package option for relevant .proto files in
that repository to reference this module.

Change-Id: Ie1ac8f81a323285efbd14a17d02c7105e810af2d
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/235283
Trust: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/integration_test.go b/integration_test.go
index 15735d9..c895e14 100644
--- a/integration_test.go
+++ b/integration_test.go
@@ -33,8 +33,8 @@
 	regenerate   = flag.Bool("regenerate", false, "regenerate files")
 	buildRelease = flag.Bool("buildRelease", false, "build release binaries")
 
-	protobufVersion = "ef7cc811" // v3.12.0-rc1
-	protobufSHA256  = ""         // ignored if protobufVersion is a git hash
+	protobufVersion = "3.15.3"
+	protobufSHA256  = "" // ignored if protobufVersion is a git hash
 
 	golangVersions = []string{"1.9.7", "1.10.8", "1.11.13", "1.12.17", "1.13.15", "1.14.7", "1.15"}
 	golangLatest   = golangVersions[len(golangVersions)-1]